JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档
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4 - JNI Functions
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:
- Version Information
- Class Operations
- Exceptions
- Global and Local References
- Object Operations
- Accessing Fields of Objects
- Calling Instance Methods
- Accessing Static Fields
- Calling Static Methods
- String Operations
- Array Operations
- Registering Native Methods
- Monitor Operations
- Java VM Interface
Interface Function Table
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:typedef const struct JNINativeInterface *JNIEnv;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.
Version Information
GetVersion
jint GetVersion(JNIEnv *env);
Returns the version of the native method interface.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the major version number in the higher 16 bits and the minor version number in the lower 16 bits.
In JDK1.1, GetVersion() returns 0x00010001.
Class Operations
DefineClass
jclass DefineClass(JNIEnv *env, jobject loader,
const jbyte *buf, jsize bufLen);
Loads a class from a buffer of raw class data.
PARAMETERS:
env: the JNI interface pointer.
loader: a class loader assigned to the defined class.
buf: buffer containing the .class file data.
RETURNS:
Returns a Java class object orNULL if an error occurs.
THROWS:
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.
FindClass
jclass FindClass(JNIEnv *env, const char *name);
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.
PARAMETERS:
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.
RETURNS:
Returns a class object from a fully-qualified name, orNULL if the class cannot
be found.
THROWS:
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.
GetSuperclass
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the superclass of the class represented byclazz, or NULL.
IsAssignableFrom
jboolean IsAssignableFrom(JNIEnv *env, jclass clazz1,
jclass clazz2);
Determines whether an object of clazz1 can be safely cast to clazz2.
PARAMETERS:
env: the JNI interface pointer.
clazz1: the first class argument.
clazz2: the second class argument.
RETURNS:
ReturnsJNI_TRUE if either of the following is true:
- The first and second class arguments refer to the same Java class.
- The first class is a subclass of the second class.
- The first class has the second class as one of its interfaces.
Exceptions
Throw
jint Throw(JNIEnv *env, jthrowable obj);
Causes a java.lang.Throwable object to be thrown.
PARAMETERS:
env: the JNI interface pointer.
obj: a java.lang.Throwable object.
RETURNS:
Returns 0 on success; a negative value on failure.THROWS:
the java.lang.Throwable object obj.
ThrowNew
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.
PARAMETERS:
env: the JNI interface pointer.
clazz: a subclass of java.lang.Throwable.
message: the message used to construct the java.lang.Throwable object.
RETURNS:
Returns 0 on success; a negative value on failure.THROWS:
the newly constructed java.lang.Throwable object.
ExceptionOccurred
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the exception object that is currently in the process of being thrown, orNULL if no exception is currently being thrown.
ExceptionDescribe
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.
PARAMETERS:
env: the JNI interface pointer.
ExceptionClear
void ExceptionClear(JNIEnv *env);
Clears any exception that is currently being thrown. If no exception is currently being thrown, this routine has no effect.
PARAMETERS:
env: the JNI interface pointer.
FatalError
void FatalError(JNIEnv *env, const char *msg);
Raises a fatal error and does not expect the VM to recover. This function does not return.
PARAMETERS:
env: the JNI interface pointer.
Global and Local References
NewGlobalRef
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().
PARAMETERS:
env: the JNI interface pointer.
obj: a global or local reference.
RETURNS:
Returns a global reference, orNULL if the system runs out of memory.
DeleteGlobalRef
void DeleteGlobalRef(JNIEnv *env, jobject globalRef);
Deletes the global reference pointed to by globalRef.
PARAMETERS:
env: the JNI interface pointer.
globalRef: a global reference.
DeleteLocalRef
void DeleteLocalRef(JNIEnv *env, jobject localRef);
Deletes the local reference pointed to by localRef.
PARAMETERS:
env: the JNI interface pointer.
Object Operations
AllocObject
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns a Java object, orNULL if the object cannot be constructed.
THROWS:
InstantiationException: if the class is an interface or an abstract class.
OutOfMemoryError: if the system runs out of memory.
NewObject
NewObjectA
NewObjectV
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.
NewObject
Programmers place all arguments that are to be passed to the constructor immediately following themethodID argument. NewObject() accepts these
arguments and passes them to the Java method that the programmer wishes to
invoke.
NewObjectA
Programmers place all arguments that are to be passed to the constructor in anargs 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.
NewObjectV
Programmers place all arguments that are to be passed to the constructor in anargs 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.
PARAMETERS:
env: the JNI interface pointer.
methodID: the method ID of the constructor.
Additional Parameter for NewObject:
arguments to the constructor.Additional Parameter for NewObjectA:
args: an array of arguments to the constructor.
Additional Parameter for NewObjectV:
args: a va_list of arguments to the constructor.
RETURNS:
Returns a Java object, orNULL if the object cannot be constructed.
THROWS:
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.
GetObjectClass
jclass GetObjectClass(JNIEnv *env, jobject obj);
Returns the class of an object.
PARAMETERS:
env: the JNI interface pointer.
obj: a Java object (must not be NULL).
RETURNS:
Returns a Java class object.IsInstanceOf
jboolean IsInstanceOf(JNIEnv *env, jobject obj,
jclass clazz);
Tests whether an object is an instance of a class.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
ReturnsJNI_TRUE if obj can be cast to clazz; otherwise, returns
JNI_FALSE. A NULL object can be cast to any class.
IsSameObject
jboolean IsSameObject(JNIEnv *env, jobject ref1,
jobject ref2);
Tests whether two references refer to the same Java object.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
ReturnsJNI_TRUE if ref1 and ref2 refer to the same Java object, or are
both NULL; otherwise, returns JNI_FALSE.
Accessing Fields of Objects
GetFieldID
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.
PARAMETERS:
env: the JNI interface pointer.
name: the field name in a 0-terminated UTF-8 string.
sig: the field signature in a 0-terminated UTF-8 string.
RETURNS:
Returns a field ID, orNULL if the operation fails.
THROWS:
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.
Get<type>Field Routines
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.
PARAMETERS:
env: the JNI interface pointer.
obj: a Java object (must not be NULL).
RETURNS:
Returns the content of the field.Set<type>Field Routines
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.
PARAMETERS:
env: the JNI interface pointer.
obj: a Java object (must not be NULL).
value: the new value of the field.
Calling Instance Methods
GetMethodID
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.
PARAMETERS:
env: the JNI interface pointer.
name: the method name in a 0-terminated UTF-8 string.
sig: the method signature in 0-terminated UTF-8 string.
RETURNS:
Returns a method ID, orNULL if the specified method cannot be found.
THROWS:
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.
Call<type>Method Routines
Call<type>MethodA Routines
Call<type>MethodV Routines
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.
Call<type>Method Routines
Programmers place all arguments that are to be passed to the method immediately following themethodID argument. The Call<type>Method routine
accepts these arguments and passes them to the Java method that the
programmer wishes to invoke.
Call<type>MethodA Routines
Programmers place all arguments to the method in anargs 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.
Call<type>MethodV Routines
Programmers place all arguments to the method in anargs 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.
PARAMETERS:
env: the JNI interface pointer.
Additional Parameter for Call<type>Method Routines:
arguments to the Java method.Additional Parameter for Call<type>MethodA Routines:
args: an array of arguments.
Additional Parameter for Call<type>MethodV Routines:
args: a va_list of arguments.
RETURNS:
Returns the result of calling the Java method.THROWS:
Exceptions raised during the execution of the Java method.
CallNonvirtual<type>Method Routines
CallNonvirtual<type>MethodA Routines
CallNonvirtual<type>MethodV Routines
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.
CallNonvirtual<type>Method Routines
Programmers place all arguments that are to be passed to the method immediately following themethodID argument. The
CallNonvirtual<type>Method routine accepts these arguments and passes them
to the Java method that the programmer wishes to invoke.
CallNonvirtual<type>MethodA Routines
Programmers place all arguments to the method in anargs 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.
CallNonvirtual<type>MethodV Routines
Programmers place all arguments to the method in anargs 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.
PARAMETERS:
env: the JNI interface pointer.
Additional Parameter for CallNonvirtual<type>Method Routines:
arguments to the Java method.Additional Parameter for CallNonvirtual<type>MethodA Routines:
args: an array of arguments.
Additional Parameter for CallNonvirtual<type>MethodV Routines:
args: a va_list of arguments.
RETURNS:
Returns the result of calling the Java method.
THROWS:
Exceptions raised during the execution of the Java method.
Accessing Static Fields
GetStaticFieldID
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.
PARAMETERS:
env: the JNI interface pointer.
name: the static field name in a 0-terminated UTF-8 string.
sig: the field signature in a 0-terminated UTF-8 string.
RETURNS:
Returns a field ID, orNULL if the specified static field cannot be found.
THROWS:
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.
GetStatic<type>Field Routines
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the content of the static field.SetStatic<type>Field Routines
void SetStatic<type>Field(JNIEnv *env, jclass clazz,
jfieldID fieldID, NativeType 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.
PARAMETERS:
env: the JNI interface pointer.
value: the new value of the field.
Calling Static Methods
GetStaticMethodID
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.
PARAMETERS:
env: the JNI interface pointer.
name: the static method name in a 0-terminated UTF-8 string.
sig: the method signature in a 0-terminated UTF-8 string.
RETURNS:
Returns a method ID, orNULL if the operation fails.
THROWS:
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.
CallStatic<type>Method Routines
CallStatic<type>MethodA Routines
CallStatic<type>MethodV Routines
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.
CallStatic<type>Method Routines
Programmers should place all arguments that are to be passed to the method immediately following themethodID argument. The CallStatic<type>Method
routine accepts these arguments and passes them to the Java method that the
programmer wishes to invoke.
CallStatic<type>MethodA Routines
Programmers should place all arguments to the method in anargs 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.
CallStatic<type>MethodV Routines
Programmers should place all arguments to the method in anargs 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.
PARAMETERS:
env: the JNI interface pointer.
Additional Parameter for CallStatic<type>Method Routines:
arguments to the static method.Additional Parameter for CallStatic<type>MethodA Routines:
args: an array of arguments.
Additional Parameter for CallStatic<type>MethodV Routines:
args: a va_list of arguments.
RETURNS:
Returns the result of calling the static Java method.THROWS:
Exceptions raised during the execution of the Java method.
String Operations
NewString
jstring NewString(JNIEnv *env, const jchar *unicodeChars,
jsize len);
Constructs a new java.lang.String object from an array of Unicode
characters.
PARAMETERS:
env: the JNI interface pointer.
unicodeChars: pointer to a Unicode string.
len: length of the Unicode string.
RETURNS:
Returns a Java string object, orNULL if the string cannot be constructed.
THROWS:
OutOfMemoryError: if the system runs out of memory.
GetStringLength
jsize GetStringLength(JNIEnv *env, jstring string);
Returns the length (the count of Unicode characters) of a Java string.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the length of the Java string.GetStringChars
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.
PARAMETERS:
env: the JNI interface pointer.
isCopy: a pointer to a boolean.
RETURNS:
Returns a pointer to a Unicode string, orNULL if the operation fails.
ReleaseStringChars
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().
PARAMETERS:
env: the JNI interface pointer.
chars: a pointer to a Unicode string.
NewStringUTF
jstring NewStringUTF(JNIEnv *env, const char *bytes);
Constructs a new java.lang.String object from an array of UTF-8
characters.
PARAMETERS:
env: the JNI interface pointer, or NULL if the string cannot be constructed.
bytes: the pointer to a UTF-8 string.
RETURNS:
Returns a Java string object, orNULL if the string cannot be constructed.
THROWS:
OutOfMemoryError: if the system runs out of memory.
GetStringUTFLength
jsize GetStringUTFLength(JNIEnv *env, jstring string);
Returns the UTF-8 length in bytes of a string.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the UTF-8 length of the string.GetStringUTFChars
const char* GetStringUTFChars(JNIEnv *env, jstring string,
jboolean *isCopy);
Returns a pointer to an array of UTF-8 characters of the string. 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.
PARAMETERS:
env: the JNI interface pointer.
isCopy: a pointer to a boolean.
RETURNS:
Returns a pointer to a UTF-8 string, orNULL if the operation fails.
ReleaseStringUTFChars
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().
PARAMETERS:
env: the JNI interface pointer.
utf: a pointer to a UTF-8 string.
Array Operations
GetArrayLength
jsize GetArrayLength(JNIEnv *env, jarray array);
Returns the number of elements in the array.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns the length of the array.NewObjectArray
jarray 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.
PARAMETERS:
env: the JNI interface pointer.
elementClass: array element class.
initialElement: initialization value.
RETURNS:
Returns a Java array object, orNULL if the array cannot be constructed.
THROWS:
OutOfMemoryError: if the system runs out of memory.
GetObjectArrayElement
jobject GetObjectArrayElement(JNIEnv *env,
jobjectArray array, jsize index);
Returns an element of an Object array.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns a Java object.THROWS:
ArrayIndexOutOfBoundsException: if index does not specify a valid
index in the array.
SetObjectArrayElement
void SetObjectArrayElement(JNIEnv *env, jobjectArray array,
jsize index, jobject value);
Sets an element of an Object array.
PARAMETERS:
env: the JNI interface pointer.
THROWS:
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.
New<PrimitiveType>Array Routines
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns a Java array, orNULL if the array cannot be constructed.
Get<PrimitiveType>ArrayElements Routines
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:
- Replace Get<PrimitiveType>ArrayElements with one of the actual primitive element accessor routine names from the table.
- Replace ArrayType with the corresponding array type.
- Replace NativeType with the corresponding native type for that routine.
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.
PARAMETERS:
env: the JNI interface pointer.
isCopy: a pointer to a boolean.
RETURNS:
Returns a pointer to the array elements, orNULL if the operation fails.
Release<PrimitiveType>ArrayElements Routines
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:
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:
- Replace Release<PrimitiveType>ArrayElements with one of the actual primitive array disposer routine names from Table 4-11.
- Replace ArrayType with the corresponding array type.
- Replace NativeType with the corresponding native type for that routine.
PARAMETERS:
env: the JNI interface pointer.
elems: a pointer to array elements.
Get<PrimitiveType>ArrayRegion Routines
void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array,
jsize start, jsize len, NativeType *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:
- Replace Get<PrimitiveType>ArrayRegion with one of the actual primitive element accessor routine names from Table 4-12.
- Replace ArrayType with the corresponding array type.
- Replace NativeType with the corresponding native type for that routine.
PARAMETERS:
env: the JNI interface pointer.
len: the number of elements to be copied.
THROWS:
ArrayIndexOutOfBoundsException: if one of the indexes in the region is
not valid.
Set<PrimitiveType>ArrayRegion Routines
void Set<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array,
jsize start, jsize len, NativeType *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:
- Replace Set<PrimitiveType>ArrayRegion with one of the actual primitive element accessor routine names from the table.
- Replace ArrayType with the corresponding array type.
- Replace NativeType with the corresponding native type for that routine.
PARAMETERS:
env: the JNI interface pointer.
len: the number of elements to be copied.
THROWS:
ArrayIndexOutOfBoundsException: if one of the indexes in the region is
not valid.
Registering Native Methods
RegisterNatives
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 nMethods parameter specifies the number of native methods in the array.
The JNINativeMethod structure is defined as follows:
typedef struct {
char *name;
char *signature;
void *fnPtr;
} JNINativeMethod;
The function pointers nominally must have the following signature:
ReturnType (*fnPtr)(JNIEnv *env, jobject objectOrClass, ...);
PARAMETERS:
env: the JNI interface pointer.
methods: the native methods in the class.
nMethods: the number of native methods in the class.
RETURNS:
Returns "0" on success; returns a negative value on failure.THROWS:
NoSuchMethodError: if a specified method cannot be found or if the method
is not native.
UnregisterNatives
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.
PARAMETERS:
env: the JNI interface pointer.
RETURNS:
Returns "0" on success; returns a negative value on failure.Monitor Operations
MonitorEnter
jint MonitorEnter(JNIEnv *env, jobject obj);
Enters the monitor associated with the underlying Java object referred to by
obj.
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.
PARAMETERS:
env: the JNI interface pointer.
obj: a normal Java object or class object.
RETURNS:
Returns "0" on success; returns a negative value on failure.MonitorExit
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.
PARAMETERS:
env: the JNI interface pointer.
obj: a normal Java object or class object.
RETURNS:
Returns "0" on success; returns a negative value on failure.Java VM Interface
GetJavaVM
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.
PARAMETERS:
env: the JNI interface pointer.
vm: a pointer to where the result should be placed.
RETURNS:
Returns "0" on success; returns a negative value on failure.
Java Native Interface Specification (HTML generated by dkramer on March 15, 1997)
Copyright © 1996, 1997 Sun Microsystems, Inc.
All rights reserved
Please send any comments or corrections to jni@java.sun.com