| Documentation Contents |
| Contents | Previous | Next |
This chapter serves as the reference section for the JNI functions. It provides a complete listing of all the JNI functions. It also presents the exact layout of the JNI function table.
Note the use of the term “must” to describe restrictions on JNI programmers. For example, when you see that a certain JNI function must receive a non-NULL object, it is your responsibility to ensure that NULL is not passed to that JNI function. As a result, a JNI implementation does not need to perform NULL pointer checks in that JNI function.
A portion of this chapter is adapted from Netscape’s JRI documentation.
The reference material groups functions by their usage. The reference section is organized by the following functional areas:
Each function is accessible at a fixed offset through the JNIEnv argument. The JNIEnv type is a pointer to a structure storing all JNI function pointers. It is defined as follows:
The VM initializes the function table, as shown by
Code Example 4-1. Note that the first three
entries are reserved for future compatibility with COM. In
addition, we reserve a number of additional NULL entries near the beginning of the function
table, so that, for example, a future class-related JNI operation
can be added after FindClass, rather than at the end of the
table.
Note that the function table can be shared among all JNI interface pointers.
const struct JNINativeInterface ... = {
NULL,
NULL,
NULL,
NULL,
GetVersion,
DefineClass,
FindClass,
FromReflectedMethod,
FromReflectedField,
ToReflectedMethod,
GetSuperclass,
IsAssignableFrom,
ToReflectedField,
Throw,
ThrowNew,
ExceptionOccurred,
ExceptionDescribe,
ExceptionClear,
FatalError,
PushLocalFrame,
PopLocalFrame,
NewGlobalRef,
DeleteGlobalRef,
DeleteLocalRef,
IsSameObject,
NewLocalRef,
EnsureLocalCapacity,
AllocObject,
NewObject,
NewObjectV,
NewObjectA,
GetObjectClass,
IsInstanceOf,
GetMethodID,
CallObjectMethod,
CallObjectMethodV,
CallObjectMethodA,
CallBooleanMethod,
CallBooleanMethodV,
CallBooleanMethodA,
CallByteMethod,
CallByteMethodV,
CallByteMethodA,
CallCharMethod,
CallCharMethodV,
CallCharMethodA,
CallShortMethod,
CallShortMethodV,
CallShortMethodA,
CallIntMethod,
CallIntMethodV,
CallIntMethodA,
CallLongMethod,
CallLongMethodV,
CallLongMethodA,
CallFloatMethod,
CallFloatMethodV,
CallFloatMethodA,
CallDoubleMethod,
CallDoubleMethodV,
CallDoubleMethodA,
CallVoidMethod,
CallVoidMethodV,
CallVoidMethodA,
CallNonvirtualObjectMethod,
CallNonvirtualObjectMethodV,
CallNonvirtualObjectMethodA,
CallNonvirtualBooleanMethod,
CallNonvirtualBooleanMethodV,
CallNonvirtualBooleanMethodA,
CallNonvirtualByteMethod,
CallNonvirtualByteMethodV,
CallNonvirtualByteMethodA,
CallNonvirtualCharMethod,
CallNonvirtualCharMethodV,
CallNonvirtualCharMethodA,
CallNonvirtualShortMethod,
CallNonvirtualShortMethodV,
CallNonvirtualShortMethodA,
CallNonvirtualIntMethod,
CallNonvirtualIntMethodV,
CallNonvirtualIntMethodA,
CallNonvirtualLongMethod,
CallNonvirtualLongMethodV,
CallNonvirtualLongMethodA,
CallNonvirtualFloatMethod,
CallNonvirtualFloatMethodV,
CallNonvirtualFloatMethodA,
CallNonvirtualDoubleMethod,
CallNonvirtualDoubleMethodV,
CallNonvirtualDoubleMethodA,
CallNonvirtualVoidMethod,
CallNonvirtualVoidMethodV,
CallNonvirtualVoidMethodA,
GetFieldID,
GetObjectField,
GetBooleanField,
GetByteField,
GetCharField,
GetShortField,
GetIntField,
GetLongField,
GetFloatField,
GetDoubleField,
SetObjectField,
SetBooleanField,
SetByteField,
SetCharField,
SetShortField,
SetIntField,
SetLongField,
SetFloatField,
SetDoubleField,
GetStaticMethodID,
CallStaticObjectMethod,
CallStaticObjectMethodV,
CallStaticObjectMethodA,
CallStaticBooleanMethod,
CallStaticBooleanMethodV,
CallStaticBooleanMethodA,
CallStaticByteMethod,
CallStaticByteMethodV,
CallStaticByteMethodA,
CallStaticCharMethod,
CallStaticCharMethodV,
CallStaticCharMethodA,
CallStaticShortMethod,
CallStaticShortMethodV,
CallStaticShortMethodA,
CallStaticIntMethod,
CallStaticIntMethodV,
CallStaticIntMethodA,
CallStaticLongMethod,
CallStaticLongMethodV,
CallStaticLongMethodA,
CallStaticFloatMethod,
CallStaticFloatMethodV,
CallStaticFloatMethodA,
CallStaticDoubleMethod,
CallStaticDoubleMethodV,
CallStaticDoubleMethodA,
CallStaticVoidMethod,
CallStaticVoidMethodV,
CallStaticVoidMethodA,
GetStaticFieldID,
GetStaticObjectField,
GetStaticBooleanField,
GetStaticByteField,
GetStaticCharField,
GetStaticShortField,
GetStaticIntField,
GetStaticLongField,
GetStaticFloatField,
GetStaticDoubleField,
SetStaticObjectField,
SetStaticBooleanField,
SetStaticByteField,
SetStaticCharField,
SetStaticShortField,
SetStaticIntField,
SetStaticLongField,
SetStaticFloatField,
SetStaticDoubleField,
NewString,
GetStringLength,
GetStringChars,
ReleaseStringChars,
NewStringUTF,
GetStringUTFLength,
GetStringUTFChars,
ReleaseStringUTFChars,
GetArrayLength,
NewObjectArray,
GetObjectArrayElement,
SetObjectArrayElement,
NewBooleanArray,
NewByteArray,
NewCharArray,
NewShortArray,
NewIntArray,
NewLongArray,
NewFloatArray,
NewDoubleArray,
GetBooleanArrayElements,
GetByteArrayElements,
GetCharArrayElements,
GetShortArrayElements,
GetIntArrayElements,
GetLongArrayElements,
GetFloatArrayElements,
GetDoubleArrayElements,
ReleaseBooleanArrayElements,
ReleaseByteArrayElements,
ReleaseCharArrayElements,
ReleaseShortArrayElements,
ReleaseIntArrayElements,
ReleaseLongArrayElements,
ReleaseFloatArrayElements,
ReleaseDoubleArrayElements,
GetBooleanArrayRegion,
GetByteArrayRegion,
GetCharArrayRegion,
GetShortArrayRegion,
GetIntArrayRegion,
GetLongArrayRegion,
GetFloatArrayRegion,
GetDoubleArrayRegion,
SetBooleanArrayRegion,
SetByteArrayRegion,
SetCharArrayRegion,
SetShortArrayRegion,
SetIntArrayRegion,
SetLongArrayRegion,
SetFloatArrayRegion,
SetDoubleArrayRegion,
RegisterNatives,
UnregisterNatives,
MonitorEnter,
MonitorExit,
GetJavaVM,
GetStringRegion,
GetStringUTFRegion,
GetPrimitiveArrayCritical,
ReleasePrimitiveArrayCritical,
GetStringCritical,
ReleaseStringCritical,
NewWeakGlobalRef,
DeleteWeakGlobalRef,
ExceptionCheck,
NewDirectByteBuffer,
GetDirectBufferAddress,
GetDirectBufferCapacity,
GetObjectRefType
};
jint GetVersion(JNIEnv
*env);
Returns the version of the native method interface.
Index 4 in the JNIEnv interface function table.
env: the JNI interface
pointer.
Returns the major version number in the higher 16 bits and the minor version number in the lower 16 bits.
In JDK/JRE 1.1, GetVersion() returns 0x00010001.
In JDK/JRE 1.2, GetVersion() returns
0x00010002.
In JDK/JRE 1.4, GetVersion() returns
0x00010004.
In JDK/JRE 1.6, GetVersion() returns
0x00010006.
#define JNI_VERSION_1_1 0x00010001 #define JNI_VERSION_1_2 0x00010002 /* Error codes */ #define JNI_EDETACHED (-2) /* thread detached from the VM */ #define JNI_EVERSION (-3) /* JNI version error
#define JNI_VERSION_1_4 0x00010004
#define JNI_VERSION_1_6 0x00010006
jclass DefineClass(JNIEnv
*env, const char *name, jobject loader,
const jbyte *buf, jsize bufLen);
Loads a class from a buffer of raw class data. The buffer containing the raw class data is not referenced by the VM after the DefineClass call returns, and it may be discarded if desired.
Index 5 in the JNIEnv interface function table.
env: the JNI interface
pointer.
name: the name of the
class or interface to be defined. The string is encoded in modified
UTF-8.
loader: a class loader
assigned to the defined class.
buf: buffer containing
the .class file data.
bufLen: buffer
length.
Returns a Java class object or NULL if an error occurs.
ClassFormatError: if
the class data does not specify a valid class.
ClassCircularityError:
if a class or interface would be its own superclass or
superinterface.
OutOfMemoryError: if
the system runs out of memory.
SecurityException: if
the caller attempts to define a class in the "java" package tree.
jclass FindClass(JNIEnv *env,
const char *name);
In JDK release 1.1, this function loads a
locally-defined class. It searches the directories and zip files
specified by the CLASSPATH environment
variable for the class with the specified name.
Since Java 2 SDK release 1.2, the Java security model allows
non-system classes to load and call native methods.
FindClass locates the class loader associated with the
current native method; that is, the class loader of the class that
declared the native method. If the native method belongs to a
system class, no class loader will be involved. Otherwise, the
proper class loader will be invoked to load and link the named
class.
Since Java 2 SDK release 1.2, when FindClass is
called through the Invocation Interface, there is no current native
method or its associated class loader. In that case, the result of
ClassLoader.getSystemClassLoader is used. This is the
class loader the virtual machine creates for applications, and is
able to locate classes listed in the java.class.path
property.
The name argument is a
fully-qualified class name or an array type signature . For
example, the fully-qualified class name for the
java.lang.String class is:
"java/lang/String"
The array type signature of the array class
java.lang.Object[] is:
"[Ljava/lang/Object;"
Index 6 in the JNIEnv interface function table.
env: the JNI interface
pointer.
name: a fully-qualified
class name (that is, a package name, delimited by
“/”, followed by the class
name). If the name begins with “[“ (the array signature character), it returns
an array class. The string is encoded in modified UTF-8.
Returns a class object from a fully-qualified
name, or NULL if the class cannot be
found.
ClassFormatError: if
the class data does not specify a valid class.
ClassCircularityError:
if a class or interface would be its own superclass or
superinterface.
NoClassDefFoundError:
if no definition for a requested class or interface can be
found.
OutOfMemoryError: if
the system runs out of memory.
jclass GetSuperclass(JNIEnv
*env, jclass clazz);
If clazz represents any
class other than the class Object, then
this function returns the object that represents the superclass of
the class specified by clazz.
If clazz specifies the
class Object, or clazz represents an interface, this function returns
NULL.
Index 10 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
Returns the superclass of the class represented by
clazz, or NULL.
jboolean
IsAssignableFrom(JNIEnv *env, jclass clazz1,
jclass clazz2);
Determines whether an object of clazz1 can be safely cast to clazz2.
Index 11 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz1: the first class
argument.
clazz2: the second
class argument.
Returns JNI_TRUE if
either of the following is true:
jint Throw(JNIEnv *env,
jthrowable obj);
Causes a java.lang.Throwable object to be thrown.
Index 13 in the JNIEnv interface function table.
env: the JNI interface
pointer.
obj: a java.lang.Throwable object.
Returns 0 on success; a negative value on failure.
the java.lang.Throwable
object obj.
jint ThrowNew(JNIEnv *env,
jclass clazz,
const char *message);
Constructs an exception object from the specified
class with the message specified by message and causes that exception to be thrown.
Index 14 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a subclass of
java.lang.Throwable.
message: the message
used to construct the java.lang.Throwable object. The string is encoded in
modified UTF-8.
Returns 0 on success; a negative value on failure.
the newly constructed java.lang.Throwable object.
jthrowable
ExceptionOccurred(JNIEnv *env);
Determines if an exception is being thrown. The
exception stays being thrown until either the native code calls
ExceptionClear(), or the Java code
handles the exception.
Index 15 in the JNIEnv interface function table.
env: the JNI interface
pointer.
Returns the exception object that is currently in
the process of being thrown, or NULL if
no exception is currently being thrown.
void ExceptionDescribe(JNIEnv
*env);
Prints an exception and a backtrace of the stack
to a system error-reporting channel, such as stderr. This is a convenience routine provided for
debugging.
Index 16 in the JNIEnv interface function table.
env: the JNI interface
pointer.
void ExceptionClear(JNIEnv
*env);
Clears any exception that is currently being thrown. If no exception is currently being thrown, this routine has no effect.
Index 17 in the JNIEnv interface function table.
env: the JNI interface
pointer.
void FatalError(JNIEnv *env,
const char *msg);
Raises a fatal error and does not expect the VM to recover. This function does not return.
Index 18 in the JNIEnv interface function table.
env: the JNI interface
pointer.
msg: an error message.
The string is encoded in modified UTF-8.
jboolean
ExceptionCheck(JNIEnv *env);
Returns
JNI_TRUEwhen there is a pending exception; otherwise, returnsJNI_FALSE.
JDK/JRE 1.2
jobject NewGlobalRef(JNIEnv
*env, jobject obj);
Creates a new global reference to the object
referred to by the obj argument. The
obj argument may be a global or local
reference. Global references must be explicitly disposed of by
calling DeleteGlobalRef().
Index 21 in the JNIEnv interface function table.
env: the JNI interface
pointer.
obj: a global or local
reference.
Returns a global reference, or NULL if the system runs out of memory.
void DeleteGlobalRef(JNIEnv
*env, jobject globalRef);
Deletes the global reference pointed to by
globalRef.
Index 22 in the JNIEnv interface function table.
env: the JNI interface
pointer.
globalRef: a global
reference.
Local references are valid for the duration of a native method call. They are freed automatically after the native method returns. Each local reference costs some amount of Java Virtual Machine resource. Programmers need to make sure that native methods do not excessively allocate local references. Although local references are automatically freed after the native method returns to Java, excessive allocation of local references may cause the VM to run out of memory during the execution of a native method.
void DeleteLocalRef(JNIEnv
*env, jobject localRef);
Deletes the local reference pointed to by
localRef.
Index 23 in the JNIEnv interface function table.
env: the JNI interface
pointer.
localRef: a local
reference.
NoteJDK/JRE 1.1 provides the As of JDK/JRE 1.2 an additional set of functions are provided for local reference lifetime management. They are the four functions listed below. |
jint EnsureLocalCapacity(JNIEnv *env, jint
capacity);
Ensures that at least a given number of local
references can be created in the current thread. Returns 0 on
success; otherwise returns a negative number and throws an
OutOfMemoryError.
Before it enters a native method, the VM automatically ensures that at least 16 local references can be created.
For backward compatibility, the VM allocates local references
beyond the ensured capacity. (As a debugging support, the VM may
give the user warnings that too many local references are being
created. In the JDK, the programmer can supply the
-verbose:jni command line option to turn on these
messages.) The VM calls FatalError if no more local
references can be created beyond the ensured capacity.
JDK/JRE 1.2
jint
PushLocalFrame(JNIEnv *env, jint capacity);
Creates a new local reference frame, in which at least a given
number of local references can be created. Returns 0 on success, a
negative number and a pending OutOfMemoryError on
failure.
Note that local references already created in previous local frames are still valid in the current local frame.
JDK/JRE 1.2
jobject
PopLocalFrame(JNIEnv *env, jobject result);
Pops off the current local reference frame, frees all the local
references, and returns a local reference in the previous local
reference frame for the given result object.
Pass NULL as result if you do not need
to return a reference to the previous frame.
JDK/JRE 1.2
jobject
NewLocalRef(JNIEnv *env, jobject ref);
Creates a new local reference that refers to the same object as
ref. The given ref may be a global or
local reference. Returns NULL if ref
refers to null.
JDK/JRE 1.2
NULL. Programmers can
detect whether a weak global reference points to a freed object by
using IsSameObject to compare the weak reference
against NULL.
Weak global references in JNI are a simplified version of the
Java Weak References, available as part of the Java 2 Platform API
( java.lang.ref package and its classes).
Clarification (added June 2001)
Since garbage collection may occur while native methods are running, objects referred to by weak global references can be freed at any time. While weak global references can be used where global references are used, it is generally inappropriate to do so, as they may become functionally equivalent to
NULLwithout notice.While
IsSameObjectcan be used to determine whether a weak global reference refers to a freed object, it does not prevent the object from being freed immediately thereafter. Consequently, programmers may not rely on this check to determine whether a weak global reference may used (as a non-NULLreference) in any future JNI function call.To overcome this inherent limitation, it is recommended that a standard (strong) local or global reference to the same object be acquired using the JNI functions
NewLocalReforNewGlobalRef, and that this strong reference be used to access the intended object. These functions will returnNULLif the object has been freed, and otherwise will return a strong reference (which will prevent the object from being freed). The new reference should be explicitly deleted when immediate access to the object is no longer required, allowing the object to be freed.The weak global reference is weaker than other types of weak references (Java objects of the SoftReference or WeakReference classes). A weak global reference to a specific object will not become functionally equivalent to
NULLuntil after SoftReference or WeakReference objects referring to that same specific object have had their references cleared.The weak global reference is weaker than Java's internal references to objects requiring finalization. A weak global reference will not become functionally equivalent to
NULLuntil after the completion of the the finalizer for the referenced object, if present.Interactions between weak global references and PhantomReferences are undefined. In particular, implementations of a Java VM may (or may not) process weak global references after PhantomReferences, and it may (or may not) be possible to use weak global references to hold on to objects which are also referred to by PhantomReference objects. This undefined use of weak global references should be avoided.
jweak NewWeakGlobalRef(JNIEnv *env, jobject
obj);
Creates a new weak global reference. Returns NULL
if obj refers to null, or if the VM runs
out of memory. If the VM runs out of memory, an
OutOfMemoryError will be thrown.
JDK/JRE 1.2
void DeleteWeakGlobalRef(JNIEnv *env, jweak
obj);
Delete the VM resources needed for the given weak global reference.
jobject AllocObject(JNIEnv
*env, jclass clazz);
Allocates a new Java object without invoking any of the constructors for the object. Returns a reference to the object.
The clazz argument must not refer to an array class.
Index 27 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
Returns a Java object, or NULL if the object cannot be constructed.
InstantiationException:
if the class is an interface or an abstract class.
OutOfMemoryError: if
the system runs out of memory.
jobject NewObject(JNIEnv *env,
jclass clazz,
jmethodID methodID, ...);
jobject NewObjectA(JNIEnv
*env, jclass clazz,
jmethodID methodID, jvalue *args);
jobject NewObjectV(JNIEnv
*env, jclass clazz,
jmethodID methodID, va_list args);
Constructs a new Java object. The method ID
indicates which constructor method to invoke. This ID must be
obtained by calling GetMethodID() with
<init> as the method name and
void (V) as
the return type.
The clazz argument must
not refer to an array class.
Programmers place all arguments that are to be
passed to the constructor immediately following the methodID argument. NewObject() accepts these arguments and passes them
to the Java method that the programmer wishes to invoke.
Index 28 in the JNIEnv interface function table.
Programmers place all arguments that are to be
passed to the constructor in an args
array of jvalues that immediately
follows the methodID argument.
NewObjectA() accepts the arguments in
this array, and, in turn, passes them to the Java method that the
programmer wishes to invoke.
Index 30 in the JNIEnv interface function table.
Programmers place all arguments that are to be
passed to the constructor in an args
argument of type va_list that
immediately follows the methodID
argument. NewObjectV() accepts these
arguments, and, in turn, passes them to the Java method that the
programmer wishes to invoke.
Index 29 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
methodID: the method ID
of the constructor.
arguments to the constructor.
args: an array of
arguments to the constructor.
args: a va_list of
arguments to the constructor.
Returns a Java object, or NULL if the object cannot be constructed.
InstantiationException:
if the class is an interface or an abstract class.
OutOfMemoryError: if
the system runs out of memory.
Any exceptions thrown by the constructor.
jclass GetObjectClass(JNIEnv
*env, jobject obj);
Returns the class of an object.
Index 31 in the JNIEnv interface function table.
env: the JNI interface
pointer.
obj: a Java object
(must not be NULL).
Returns a Java class object.
jobjectRefType GetObjectRefType(JNIEnv* env, jobject
obj);
Returns the type of the object referred to
by the obj argument. The argument obj can
either be a local, global or weak global reference.
Index 232 in the JNIEnv interface function table.
env: the JNI interface pointer.
obj: a local, global or weak global reference.
vm: the virtual machine instance from
which the interface will be retrieved.
env: pointer to the location where the
JNI interface pointer for the current thread will be placed.
version: the requested JNI
version.
The function GetObjectRefType returns one of the following
enumerated values defined as a jobjectRefType:
JNIInvalidRefType = 0,
JNILocalRefType = 1,
JNIGlobalRefType = 2,
JNIWeakGlobalRefType = 3
If the argument obj is a weak
global reference type, the return will be JNIWeakGlobalRefType.
If the argument obj is a global
reference type, the return value will be JNIGlobalRefType.
If the argument obj is a local
reference type, the return will be JNILocalRefType.
If the obj argument is not a
valid reference, the return value for this function will be
JNIInvalidRefType.
An invalid reference is a reference which is
not a valid handle. That is, the obj pointer address
does not point to a location in memory which has been allocated
from one of the Ref creation functions or returned from a JNI
function.
As such, NULL
would be an invalid reference and GetObjectRefType(env,NULL) would return JNIInvalidRefType.
On the other hand, a null reference, which is a reference that points to a null, would return the type of reference that the null reference was originally created as.
GetObjectRefType
cannot be used on deleted references.
Since references are typically implemented
as pointers to memory data structures that can potentially be
reused by any of the reference allocation services in the VM, once
deleted, it is not specified what value the GetObjectRefType will return.
JDK/JRE 1.6
jboolean IsInstanceOf(JNIEnv
*env, jobject obj,
jclass clazz);
Tests whether an object is an instance of a class.
Index 32 in the JNIEnv interface function table.
env: the JNI interface
pointer.
obj: a Java object.
clazz: a Java class
object.
Returns JNI_TRUE if
obj can be cast to clazz; otherwise, returns JNI_FALSE. A NULL object
can be cast to any class.
jboolean IsSameObject(JNIEnv
*env, jobject ref1,
jobject ref2);
Tests whether two references refer to the same Java object.
Index 24 in the JNIEnv interface function table.
env: the JNI interface
pointer.
ref1: a Java
object.
ref2: a Java
object.
Returns JNI_TRUE if
ref1 and ref2
refer to the same Java object, or are both NULL; otherwise, returns JNI_FALSE.
jfieldID GetFieldID(JNIEnv
*env, jclass clazz,
const char *name, const char *sig);
Returns the field ID for an instance (nonstatic) field of a class. The field is specified by its name and signature. The Get<type>Field and Set<type>Field families of accessor functions use field IDs to retrieve object fields.
GetFieldID() causes an
uninitialized class to be initialized.
GetFieldID() cannot be
used to obtain the length field of an array. Use GetArrayLength() instead.
Index 94 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
name: the field name in
a 0-terminated modified UTF-8 string.
sig: the field
signature in a 0-terminated modified UTF-8 string.
Returns a field ID, or NULL if the operation fails.
NoSuchFieldError: if
the specified field cannot be found.
ExceptionInInitializerError: if the class
initializer fails due to an exception.
OutOfMemoryError: if
the system runs out of memory.
NativeType Get<type>Field(JNIEnv
*env, jobject obj,
jfieldID fieldID);
This family of accessor routines returns the value
of an instance (nonstatic) field of an object. The field to access
is specified by a field ID obtained by calling GetFieldID().
The following table describes the Get<type>Field routine name and result type. You should replace type in Get<type>Field with the Java type of the field, or use one of the actual routine names from the table, and replace NativeType with the corresponding native type for that routine.
env: the JNI interface
pointer.
obj: a Java object
(must not be NULL).
fieldID: a valid field
ID.
Returns the content of the field.
void Set<type>Field(JNIEnv
*env, jobject obj, jfieldID fieldID, NativeType value);
This family of accessor routines sets the value of
an instance (nonstatic) field of an object. The field to access is
specified by a field ID obtained by calling GetFieldID().
The following table describes the Set<type>Field routine name and value type. You should replace type in Set<type>Field with the Java type of the field, or use one of the actual routine names from the table, and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
obj: a Java object
(must not be NULL).
fieldID: a valid field
ID.
value: the new value of
the field.
jmethodID GetMethodID(JNIEnv
*env, jclass clazz,
const char *name, const char *sig);
Returns the method ID for an instance (nonstatic)
method of a class or interface. The method may be defined in one of
the clazz’s superclasses and
inherited by clazz. The method is
determined by its name and signature.
GetMethodID() causes an
uninitialized class to be initialized.
To obtain the method ID of a constructor, supply
<init> as the method name and
void (V) as
the return type.
Index 33 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
name: the method name
in a 0-terminated modified UTF-8 string.
sig: the method
signature in 0-terminated modified UTF-8 string.
Returns a method ID, or NULL if the specified method cannot be found.
NoSuchMethodError: if
the specified method cannot be found.
ExceptionInInitializerError: if the class
initializer fails due to an exception.
OutOfMemoryError: if
the system runs out of memory.
NativeType Call<type>Method(JNIEnv
*env, jobject obj,
jmethodID methodID, ...);
NativeType Call<type>MethodA(JNIEnv
*env, jobject obj,
jmethodID methodID, jvalue *args);
NativeType Call<type>MethodV(JNIEnv
*env, jobject obj,
jmethodID methodID, va_list args);
Methods from these three families of operations are used to call a Java instance method from a native method.They only differ in their mechanism for passing parameters to the methods that they call.
These families of operations invoke an instance
(nonstatic) method on a Java object, according to the specified
method ID. The methodID argument must be
obtained by calling GetMethodID().
When these functions are used to call private
methods and constructors, the method ID must be derived from the
real class of obj, not from one of its
superclasses.
Programmers place all arguments that are to be
passed to the method immediately following the methodID argument. The Call<type>Method routine accepts these
arguments and passes them to the Java method that the programmer
wishes to invoke.
Programmers place all arguments to the method in
an args array of jvalues that immediately follows the methodID argument. The Call<type>MethodA routine accepts the
arguments in this array, and, in turn, passes them to the Java
method that the programmer wishes to invoke.
Programmers place all arguments to the method in
an args argument of type va_list that immediately follows the methodID argument. The Call<type>MethodV routine accepts the
arguments, and, in turn, passes them to the Java method that the
programmer wishes to invoke.
The following table describes each of the method calling routines according to their result type. You should replace type in Call<type>Method with the Java type of the method you are calling (or use one of the actual method calling routine names from the table) and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table:
env: the JNI interface
pointer.
obj: a Java object.
methodID: a method
ID.
arguments to the Java method.
args: an array of
arguments.
args: a va_list of
arguments.
Returns the result of calling the Java method.
Exceptions raised during the
execution of the Java method.
NativeType CallNonvirtual<type>Method(JNIEnv *env, jobject obj,
jclass clazz, jmethodID methodID, ...);
NativeType CallNonvirtual<type>MethodA(JNIEnv *env, jobject obj,
jclass clazz, jmethodID methodID, jvalue *args);
NativeType CallNonvirtual<type>MethodV(JNIEnv *env, jobject obj,
jclass clazz, jmethodID methodID, va_list args);
These families of operations invoke an instance
(nonstatic) method on a Java object, according to the specified
class and method ID. The methodID
argument must be obtained by calling GetMethodID() on the
class clazz.
The CallNonvirtual<type>Method families of
routines and the Call<type>Method
families of routines are different. Call<type>Method routines invoke the method
based on the class of the object, while CallNonvirtual<type>Method routines invoke
the method based on the class, designated by the clazz parameter, from which the method ID is
obtained. The method ID must be obtained from the real class of the
object or from one of its superclasses.
Programmers place all arguments that are to be
passed to the method immediately following the methodID argument. The CallNonvirtual<type>Method routine accepts
these arguments and passes them to the Java method that the
programmer wishes to invoke.
Programmers place all arguments to the method in
an args array of jvalues that immediately follows the methodID argument. The CallNonvirtual<type>MethodA routine accepts
the arguments in this array, and, in turn, passes them to the Java
method that the programmer wishes to invoke.
Programmers place all arguments to the method in
an args argument of type va_list that immediately follows the methodID argument. The CallNonvirtualMethodV routine accepts the
arguments, and, in turn, passes them to the Java method that the
programmer wishes to invoke.
The following table describes each of the method calling routines according to their result type. You should replace type in CallNonvirtual<type>Method with the Java type of the method, or use one of the actual method calling routine names from the table, and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class.
obj: a Java object.
methodID: a method
ID.
arguments to the Java method.
args: an array of
arguments.
args: a va_list of arguments.
Returns the result of calling the Java method.
Exceptions raised during the execution of the Java method.
jfieldID
GetStaticFieldID(JNIEnv *env, jclass clazz,
const char *name, const char *sig);
Returns the field ID for a static field of a class. The field is specified by its name and signature. The GetStatic<type>Field and SetStatic<type>Field families of accessor functions use field IDs to retrieve static fields.
GetStaticFieldID()
causes an uninitialized class to be initialized.
Index 144 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
name: the static field
name in a 0-terminated modified UTF-8 string.
sig: the field
signature in a 0-terminated modified UTF-8 string.
Returns a field ID, or NULL if the specified static field cannot be
found.
NoSuchFieldError: if
the specified static field cannot be found.
ExceptionInInitializerError: if the class
initializer fails due to an exception.
OutOfMemoryError: if
the system runs out of memory.
NativeType GetStatic<type>Field(JNIEnv *env, jclass clazz,
jfieldID fieldID);
This family of accessor routines returns the value
of a static field of an object. The field to access is specified by
a field ID, which is obtained by calling GetStaticFieldID().
The following table describes the family of get routine names and result types. You should replace type in GetStatic<type>Field with the Java type of the field, or one of the actual static field accessor routine names from the table, and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
fieldID: a static field
ID.
Returns the content of the static field.
void SetStatic<type>Field(JNIEnv *env, jclass clazz, NativeType
jfieldID fieldID,value);
This family of accessor routines sets the value of
a static field of an object. The field to access is specified by a
field ID, which is obtained by calling GetStaticFieldID().
The following table describes the set routine name and value types. You should replace type in SetStatic<type>Field with the Java type of the field, or one of the actual set static field routine names from the table, and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
fieldID: a static field
ID.
value: the new value of
the field.
jmethodID
GetStaticMethodID(JNIEnv *env, jclass clazz,
const char *name, const char *sig);
Returns the method ID for a static method of a class. The method is specified by its name and signature.
GetStaticMethodID()
causes an uninitialized class to be initialized.
env: the JNI interface
pointer.
clazz: a Java class
object.
name: the static method
name in a 0-terminated modified UTF-8 string.
sig: the method
signature in a 0-terminated modified UTF-8 string.
Returns a method ID, or NULL if the operation fails.
NoSuchMethodError: if
the specified static method cannot be found.
ExceptionInInitializerError: if the class
initializer fails due to an exception.
OutOfMemoryError: if
the system runs out of memory.
NativeType CallStatic<type>Method(JNIEnv *env, jclass clazz,
jmethodID methodID, ...);
NativeType CallStatic<type>MethodA(JNIEnv *env, jclass clazz,
jmethodID methodID, jvalue *args);
NativeType CallStatic<type>MethodV(JNIEnv *env, jclass clazz,
jmethodID methodID, va_list args);
This family of operations invokes a static method
on a Java object, according to the specified method ID. The
methodID argument must be obtained by
calling GetStaticMethodID().
The method ID must be derived from clazz, not from one of its superclasses.
Programmers should place all arguments that are to
be passed to the method immediately following the methodID argument. The CallStatic<type>Method routine accepts these
arguments and passes them to the Java method that the programmer
wishes to invoke.
Programmers should place all arguments to the
method in an args array of jvalues that immediately follows the methodID argument. The CallStaticMethodA routine accepts the arguments in
this array, and, in turn, passes them to the Java method that the
programmer wishes to invoke.
Programmers should place all arguments to the
method in an args argument of type
va_list that immediately follows the
methodID argument. The CallStaticMethodV routine accepts the arguments,
and, in turn, passes them to the Java method that the programmer
wishes to invoke.
The following table describes each of the method calling routines according to their result types. You should replace type in CallStatic<type>Method with the Java type of the method, or one of the actual method calling routine names from the table, and replace NativeType with the corresponding native type for that routine.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
methodID: a static
method ID.
arguments to the static method.
args: an array of
arguments.
args: a va_list of arguments.
Returns the result of calling the static Java method.
Exceptions raised during the
execution of the Java method.
jstring NewString(JNIEnv *env,
const jchar *unicodeChars,
jsize len);
Constructs a new java.lang.String object from an array of Unicode
characters.
env: the JNI interface
pointer.
unicodeChars: pointer
to a Unicode string.
len: length of the
Unicode string.
Returns a Java string object, or NULL if the string cannot be constructed.
OutOfMemoryError: if
the system runs out of memory.
jsize GetStringLength(JNIEnv
*env, jstring string);
Returns the length (the count of Unicode characters) of a Java string.
env: the JNI interface
pointer.
string: a Java string
object.
Returns the length of the Java string.
const jchar *
GetStringChars(JNIEnv *env, jstring string,
jboolean *isCopy);
Returns a pointer to the array of Unicode
characters of the string. This pointer is valid until ReleaseStringchars() is called.
If isCopy is not
NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to
JNI_FALSE if no copy is made.
env: the JNI interface
pointer.
string: a Java string
object.
isCopy: a pointer to a
boolean.
Returns a pointer to a Unicode string, or
NULL if the operation fails.
void ReleaseStringChars(JNIEnv
*env, jstring string,
const jchar *chars);
Informs the VM that the native code no longer
needs access to chars. The chars argument is a pointer obtained from
string using GetStringChars().
env: the JNI interface
pointer.
string: a Java string
object.
chars: a pointer to a
Unicode string.
jstring NewStringUTF(JNIEnv
*env, const char *bytes);
Constructs a new java.lang.String object from an array of characters
in modified UTF-8 encoding.
env: the JNI interface
pointer.
bytes: the pointer to a
modified UTF-8 string.
Returns a Java string object, or NULL if the string cannot be constructed.
OutOfMemoryError: if
the system runs out of memory.
jsize
GetStringUTFLength(JNIEnv *env, jstring string);
Returns the length in bytes of the modified UTF-8 representation of a string.
env: the JNI interface
pointer.
string: a Java string
object.
Returns the UTF-8 length of the string.
const char *
GetStringUTFChars(JNIEnv *env, jstring string,
jboolean *isCopy);
Returns a pointer to an array of bytes
representing the string in modified UTF-8 encoding. This array is
valid until it is released by ReleaseStringUTFChars().
If isCopy is not
NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to
JNI_FALSE if no copy is made.
env: the JNI interface
pointer.
string: a Java string
object.
isCopy: a pointer to a
boolean.
Returns a pointer to a modified UTF-8 string, or
NULL if the operation fails.
void
ReleaseStringUTFChars(JNIEnv *env, jstring string,
const char *utf);
Informs the VM that the native code no longer
needs access to utf. The utf argument is a pointer derived from string using GetStringUTFChars().
env: the JNI interface
pointer.
string: a Java string
object.
utf: a pointer to a
modified UTF-8 string.
NoteIn JDK/JRE 1.1, programmers can get primitive array elements in a user-supplied buffer. As of JDK/JRE 1.2 additional set of functions are provided allowing native code to obtain characters in Unicode (UTF-16) or modified UTF-8 encoding in a user-supplied buffer. See the functions below. |
void GetStringRegion(JNIEnv *env, jstring str, jsize
start, jsize len, jchar *buf);
Copies len number of Unicode characters beginning
at offset start to the given buffer
buf.
Throws StringIndexOutOfBoundsException on index
overflow.
JDK/JRE 1.2
< void GetStringUTFRegion(JNIEnv *env, jstring str,
jsize start, jsize len, char *buf);
Translates len number of Unicode characters
beginning at offset start into modified UTF-8 encoding
and place the result in the given buffer buf.
Throws StringIndexOutOfBoundsException on index
overflow.
JDK/JRE 1.2
const jchar * GetStringCritical(JNIEnv *env, jstring
string, jboolean *isCopy);
void ReleaseStringCritical(JNIEnv *env, jstring string, const jchar
*carray);
The semantics of these two functions are similar to the existing
Get/ReleaseStringChars functions. If possible, the VM
returns a pointer to string elements; otherwise, a copy is made.
However, there are significant restrictions on how these
functions can be used. In a code segment enclosed by
Get/ReleaseStringCritical calls, the native code must
not issue arbitrary JNI calls, or cause the current thread to
block.
The restrictions on Get/ReleaseStringCritical are
similar to those on
Get/ReleasePrimitiveArrayCritical.
JDK/JRE 1.2
jsize GetArrayLength(JNIEnv
*env, jarray array);
Returns the number of elements in the array.
env: the JNI interface
pointer.
array: a Java array
object.
Returns the length of the array.
jobjectArray
NewObjectArray(JNIEnv *env, jsize length,
jclass elementClass, jobject initialElement);
Constructs a new array holding objects in class
elementClass. All elements are initially
set to initialElement.
env: the JNI interface
pointer.
length: array size.
elementClass: array
element class.
initialElement:
initialization value.
Returns a Java array object, or NULL if the array cannot be constructed.
OutOfMemoryError: if
the system runs out of memory.
jobject
GetObjectArrayElement(JNIEnv *env,
jobjectArray array, jsize index);
Returns an element of an Object array.
env: the JNI interface
pointer.
array: a Java
array.
index: array index.
Returns a Java object.
ArrayIndexOutOfBoundsException: if index does not specify a valid index in the
array.
void
SetObjectArrayElement(JNIEnv *env, jobjectArray array,
jsize index, jobject value);
Sets an element of an Object array.
env: the JNI interface
pointer.
array: a Java
array.
index: array index.
value: the new
value.
ArrayIndexOutOfBoundsException: if index does not specify a valid index in the
array.
ArrayStoreException: if
the class of value is not a subclass of
the element class of the array.
ArrayType New<PrimitiveType>Array(JNIEnv *env, jsize length);
A family of operations used to construct a new primitive array object. Table 4-8 describes the specific primitive array constructors. You should replace New<PrimitiveType>Array with one of the actual primitive array constructor routine names from the following table, and replace ArrayType with the corresponding array type for that routine.
Indices in the JNIEnv interface function table.
|
New<PrimitiveType>Array Routines
|
Index
|
|---|---|
NewBooleanArray() |
175
|
NewByteArray() |
176
|
NewCharArray() |
177
|
NewShortArray() |
178
|
NewIntArray() |
179
|
NewLongArray() |
180
|
NewFloatArray() |
181
|
NewDoubleArray() |
182
|
env: the JNI interface
pointer.
length: the array
length.
Returns a Java array, or NULL if the array cannot be constructed.
NativeType *Get<PrimitiveType>ArrayElements(JNIEnv *env,
ArrayType
array, jboolean *isCopy);
A family of functions that returns the body of the
primitive array. The result is valid until the corresponding
Release<PrimitiveType>ArrayElements()
function is called. Since the returned array
may be a copy of the Java array, changes made to the returned array
will not necessarily be reflected in the original array until Release<PrimitiveType>ArrayElements()
is called.
If isCopy is not
NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to
JNI_FALSE if no copy is made.
The following table describes the specific primitive array element accessors. You should make the following substitutions:
Regardless of how boolean arrays are represented
in the Java VM, GetBooleanArrayElements() always returns a pointer
to jbooleans, with each byte denoting an
element (the unpacked representation). All arrays of other types
are guaranteed to be contiguous in memory.
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
array: a Java string
object.
isCopy: a pointer to a
boolean.
Returns a pointer to the array elements, or
NULL if the operation fails.
void Release<PrimitiveType>ArrayElements(JNIEnv *env, ArrayType array, NativeType
*elems, jint mode);
A family of functions that informs the VM that the
native code no longer needs access to elems. The elems argument
is a pointer derived from array using
the corresponding Get<PrimitiveType>ArrayElements()
function. If necessary, this function copies back all changes made
to elems to the original array.
The mode argument
provides information on how the array buffer should be released.
mode has no effect if elems is not a copy of the elements in array. Otherwise, mode
has the following impact, as shown in the following table:
|
mode
|
actions
|
|---|---|
0 |
copy back the content and free the
elems buffer |
JNI_COMMIT |
copy back the content but do not free the
elems buffer |
JNI_ABORT |
free the buffer without copying back the
possible changes
|
In most cases, programmers pass “0” to
the mode argument to ensure consistent
behavior for both pinned and copied arrays. The other options give
the programmer more control over memory management and should be
used with extreme care.
The next table describes the specific routines that comprise the family of primitive array disposers. You should make the following substitutions:
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
array: a Java array
object.
elems: a pointer to
array elements.
mode: the release
mode.
void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType
array, NativeType
jsize start, jsize len,*buf);
A family of functions that copies a region of a primitive array into a buffer.
The following table describes the specific primitive array element accessors. You should do the following substitutions:
Indices in the JNIEnv interface function table.
env: the JNI interface
pointer.
array: a Java
array.
start: the starting
index.
len: the number of
elements to be copied.
buf: the destination
buffer.
ArrayIndexOutOfBoundsException: if one of the
indexes in the region is not valid.
void Set<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType
array, NativeType
jsize start, jsize len,*buf);
A family of functions that copies back a region of a primitive array from a buffer.
The following table describes the specific primitive array element accessors. You should make the following replacements:
Indices in the JNIEnv interface function table.
PARAMETERS:
env: the JNI interface
pointer.
array: a Java
array.
start: the starting
index.
len: the number of
elements to be copied.
buf: the source
buffer.
ArrayIndexOutOfBoundsException: if one of the
indexes in the region is not valid.
NoteAs of JDK/JRE 1.1, programmers can use
New functions introduced as of JDK/JRE 1.3 allow native code to obtain a direct pointer to array elements even if the VM does not support pinning. |
void *
GetPrimitiveArrayCritical(JNIEnv *env, jarray array, jboolean
*isCopy);
void
ReleasePrimitiveArrayCritical(JNIEnv *env, jarray array, void
*carray, jint mode);
The semantics of these two functions are very similar to the
existing Get/Release<primitivetype>ArrayElements
functions. If possible, the VM returns a pointer to the primitive
array; otherwise, a copy is made. However, there are
significant restrictions on how these functions can be
used.
After calling GetPrimitiveArrayCritical, the native
code should not run for an extended period of time before it calls
ReleasePrimitiveArrayCritical. We must treat the code
inside this pair of functions as running in a "critical region."
Inside a critical region, native code must not call other JNI
functions, or any system call that may cause the current thread to
block and wait for another Java thread. (For example, the current
thread must not call read on a stream being written by
another Java thread.)
These restrictions make it more likely that the native
code will obtain an uncopied version of the array, even if the VM
does not support pinning. For example, a VM may
temporarily disable garbage collection when the native code is
holding a pointer to an array obtained via
GetPrimitiveArrayCritical.
Multiple pairs of GetPrimtiveArrayCritical and
ReleasePrimitiveArrayCritical may be nested. For
example:
jint len = (*env)->GetArrayLength(env, arr1);
jbyte *a1 = (*env)->GetPrimitiveArrayCritical(env, arr1, 0);
jbyte *a2 = (*env)->GetPrimitiveArrayCritical(env, arr2, 0);
/* We need to check in case the VM tried to make a copy. */
if (a1 == NULL || a2 == NULL) {
... /* out of memory exception thrown */
}
memcpy(a1, a2, len);
(*env)->ReleasePrimitiveArrayCritical(env, arr2, a2, 0);
(*env)->ReleasePrimitiveArrayCritical(env, arr1, a1, 0);
Note that GetPrimitiveArrayCritical might still
make a copy of the array if the VM internally represents arrays in
a different format. Therefore we need to check its return value
against NULL for possible out of memory situations.
Linkage Index 222 in the JNIEnv interface function table.
Linkage Index 223 in the JNIEnv interface function table.
JDK/JRE 1.2
jint RegisterNatives(JNIEnv
*env, jclass clazz,
const JNINativeMethod *methods, jint nMethods);
Registers native methods with the class specified
by the clazz argument. The methods parameter specifies an array of JNINativeMethod structures that contain the names,
signatures, and function pointers of the native methods. The
name and signature fields of the JNINativeMethod structure
are pointers to modified UTF-8 strings. The nMethods parameter specifies the number of native
methods in the array. The JNINativeMethod structure is defined as follows:
The function pointers nominally must have the following signature:
Index 215 in the JNIEnv interface function table.
env: the JNI interface
pointer.
clazz: a Java class
object.
methods: the native
methods in the class.
nMethods: the number of
native methods in the class.
Returns “0” on success; returns a negative value on failure.
NoSuchMethodError: if a
specified method cannot be found or if the method is not
native.
jint UnregisterNatives(JNIEnv
*env, jclass clazz);
Unregisters native methods of a class. The class goes back to the state before it was linked or registered with its native method functions.
This function should not be used in normal native code. Instead, it provides special programs a way to reload and relink native libraries.
env: the JNI interface
pointer.
clazz: a Java class
object.
Returns “0” on success; returns a negative value on failure.
jint MonitorEnter(JNIEnv *env,
jobject obj);
Enters the monitor associated with the underlying
Java object referred to by obj.
obj reference must not be NULL.
Each Java object has a monitor associated with it.
If the current thread already owns the monitor associated with
obj, it increments a counter in the
monitor indicating the number of times this thread has entered the
monitor. If the monitor associated with obj is not owned by any thread, the current thread
becomes the owner of the monitor, setting the entry count of this
monitor to 1. If another thread already owns the monitor associated
with obj, the current thread waits until
the monitor is released, then tries again to gain ownership.
A monitor entered through a
MonitorEnter JNI function call cannot be exited using
the monitorexit Java virtual machine instruction or a
synchronized method return. A MonitorEnter JNI
function call and a monitorenter Java virtual machine
instruction may race to enter the monitor associated with the same
object.
To avoid deadlocks, a monitor entered
through a MonitorEnter JNI function call must be
exited using the MonitorExit JNI call, unless the
DetachCurrentThread call is used to implicitly release
JNI monitors.
env: the JNI interface
pointer.
obj: a normal Java
object or class object.
Returns “0” on success; returns a negative value on failure.
jint MonitorExit(JNIEnv *env,
jobject obj);
The current thread must be the owner of the
monitor associated with the underlying Java object referred to by
obj. The thread decrements the counter
indicating the number of times it has entered this monitor. If the
value of the counter becomes zero, the current thread releases the
monitor.
Native code must not use
MonitorExit to exit a monitor entered through a
synchronized method or a monitorenter Java virtual
machine instruction.
env: the JNI interface
pointer.
obj: a normal Java
object or class object.
Returns “0” on success; returns a negative value on failure.
IllegalMonitorStateException: if the current
thread does not own the monitor.
The NIO-related entry points allow native code to access
java.nio direct buffers. The contents of a
direct buffer can, potentially, reside in native memory outside of
the ordinary garbage-collected heap. For information about direct
buffers, please see New I/O APIs
and the specification of the java.nio.ByteBuffer
class.
Every implementation of the Java virtual machine must support these functions, but not every implementation is required to support JNI access to direct buffers. If a JVM does not support such access then the NewDirectByteBuffer and GetDirectBufferAddress functions must always return NULL, and the GetDirectBufferCapacity function must always return -1. If a JVM does support such access then these three functions must be implemented to return the appropriate values.
jobject NewDirectByteBuffer(JNIEnv* env, void* address, jlong capacity);
Allocates and returns a direct java.nio.ByteBuffer referring to the block of memory starting at the memory address address and extending capacity bytes.
Native code that calls this function and returns the resulting byte-buffer object to Java-level code should ensure that the buffer refers to a valid region of memory that is accessible for reading and, if appropriate, writing. An attempt to access an invalid memory location from Java code will either return an arbitrary value, have no visible effect, or cause an unspecified exception to be thrown.
Index 229 in the JNIEnv interface function table.
env: the JNIEnv interface pointer
address: the starting address of the memory region (must not be NULL)
capacity: the size in bytes of the memory region (must be positive)
Returns a local reference to the newly-instantiated java.nio.ByteBuffer object. Returns NULL if an exception occurs, or if JNI access to direct buffers is not supported by this virtual machine.
OutOfMemoryError: if allocation of the ByteBuffer object fails
JDK/JRE 1.4
void* GetDirectBufferAddress(JNIEnv* env, jobject buf);
Fetches and returns the starting address of the memory region referenced by the given direct java.nio.Buffer.
This function allows native code to access the same memory region that is accessible to Java code via the buffer object.
Index 230 in the JNIEnv interface function table.
env: the JNIEnv interface pointer
buf: a direct java.nio.Buffer object (must not be NULL)
Returns the starting address of the memory region referenced by the buffer. Returns NULL if the memory region is undefined, if the given object is not a direct java.nio.Buffer, or if JNI access to direct buffers is not supported by this virtual machine.
JDK/JRE 1.4
jlong GetDirectBufferCapacity(JNIEnv* env, jobject buf);
Fetches and returns the capacity in bytes of the memory region referenced by the given direct java.nio.Buffer.
Index 231 in the JNIEnv interface function table.
env: the JNIEnv interface pointer
buf: a direct java.nio.Buffer object (must not be NULL)
Returns the capacity in bytes of the memory region associated with the buffer. Returns -1 if the given object is not a direct java.nio.Buffer, if the object is an unaligned view buffer and the processor architecture does not support unaligned access, or if JNI access to direct buffers is not supported by this virtual machine.
JDK/JRE 1.4
Programmers can use the JNI to call Java methods or access Java fields if they know the name and type of the methods or fields. The Java Core Reflection API allows programmers to introspect Java classes at runtime. JNI provides a set of conversion functions between field and method IDs used in the JNI to field and method objects used in the Java Core Reflection API.
jmethodID FromReflectedMethod(JNIEnv *env, jobject
method);
Converts a java.lang.reflect.Method or
java.lang.reflect.Constructor object to a method
ID.
Index 7 in the JNIEnv interface function table.
JDK/JRE 1.2
jfieldID FromReflectedField(JNIEnv *env, jobject
field);
Converts a java.lang.reflect.Field to a field
ID.
JDK/JRE 1.2
jobject ToReflectedMethod(JNIEnv *env, jclass cls,
jmethodID methodID);
Converts a method ID derived from cls to a
java.lang.reflect.Method or
java.lang.reflect.Constructor object.
Throws OutOfMemoryError and returns 0 if fails.
Index 9 in the JNIEnv interface function table.
JDK/JRE 1.2
jobject ToReflectedField(JNIEnv *env, jclass cls,
jfieldID fieldID);
Converts a field ID derived from cls to a
java.lang.reflect.Field object.
Throws OutOfMemoryError and returns 0 if fails.
Index 12 in the JNIEnv interface function table.
JDK/JRE 1.2
jint GetJavaVM(JNIEnv *env,
JavaVM **vm);
Returns the Java VM interface (used in the
Invocation API) associated with the current thread. The result is
placed at the location pointed to by the second argument,
vm.
Index 219 in the JNIEnv interface function table.
env: the JNI interface
pointer.
vm: a pointer to where
the result should be placed.
Returns “0” on success; returns a negative value on failure.
| Contents | Previous | Next |
Copyright © 1993, 2011, Oracle and/or its affiliates. All rights reserved. Please send comments using this Feedback page. |
 Java Technology |