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JNI_CreateJavaVM
jint JNI_CreateJavaVM(JavaVM **p_vm, JNIEnv **p_env, void *vm_args);
Loads and initializes a Java VM. The current thread becomes the main thread. Sets the env argument to the JNI interface pointer of the main thread.
JDK 1.1 does not support creating more than one VM in a single process. The version field in vm_args must2 be set to 0x00010001.
In JDK 1.1, the second argument to JNI_CreateJavaVM is always a pointer to JNIEnv *. The third argument is a pointer to a JDK 1.1 specific structure (JDK1_1InitArgs). The JDK1_1InitArgs structure is clearly not designed to be portable on all VMs.
In the JDK, we introduce a standard VM initialization structure. Backward compatibility is preserved. If the VM initialization argument points to a JDK1_1InitArgs structure, JNI_CreateJavaVM still returns the 1.1 version of JNI interface pointer. The VM returns the 1.2 version of JNI interface pointer if the third argument points to a JavaVMInitArgs structure. Unlike JDK1_1InitArgs, which contains a fixed set of options, JavaVMInitArgs uses option strings to encode arbitrary VM start up options.
typedef struct JavaVMInitArgs {
jint version;
jint nOptions;
JavaVMOption *options;
jboolean ignoreUnrecognized;
} JavaVMInitArgs;
The version field must be set to JNI_VERSION_1_2. (In contrast, the version field in JDK1_1InitArgs must be set to JNI_VERSION_1_1.) The options field is an array of the following type:
typedef struct JavaVMOption {
char *optionString; /* the option as a string in the default platform encoding */
void *extraInfo;
} JavaVMOption;
The size of the array is denoted by the nOptions field in JavaVMInitArgs. If ignoreUnrecognized is JNI_TRUE, JNI_CreateJavaVM ignore all unrecognized option strings that begin with "-X" or "_". If ignoreUnrecognized is JNI_FALSE, JNI_CreateJavaVM returns JNI_ERR as soon as it encounters any unrecognized option strings. All Java VMs must recognize the following set of standard options:
optionString
meaning
-D<name>=<value> Set a system property
-verbose[:class|gc|jni] Enable verbose output. The options can be followed by a comma-separated list of names indicating what kind of messages will be printed by the VM. For example, "-verbose:gc,class" instructs the VM to print GC and class loading related messages. Standard names include: gc, class, and jni. All nonstandard (VM-specific) names must begin with "X".
vfprintf extraInfo is a pointer to the vfprintf hook.
exit extraInfo is a pointer to the exit hook.
abort extraInfo is a pointer to the abort hook.
In addition, each VM implementation may support its own set of non-standard option strings. Non-standard option names must begin with "-X" or an underscore ("_"). For example, the JDK supports -Xms and -Xmx options to allow programmers specify the initial and maximum heap size. Options that begin with "-X" are accessible from the "java" command line.
Here is the example code that creates a Java VM in the JDK:
JavaVMInitArgs vm_args;
JavaVMOption options[4];
options[0].optionString = "-Djava.compiler=NONE"; /* disable JIT */
options[1].optionString = "-Djava.class.path=c:\myclasses"; /* user classes */
options[2].optionString = "-Djava.library.path=c:\mylibs"; /* set native library path */
options[3].optionString = "-verbose:jni"; /* print JNI-related messages */
vm_args.version = JNI_VERSION_1_2;
vm_args.options = options;
vm_args.nOptions = 4;
vm_args.ignoreUnrecognized = TRUE;
/* Note that in the JDK, there is no longer any need to call
* JNI_GetDefaultJavaVMInitArgs.
*/
res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);
if (res < 0) ...
The JDK still supports JDK1_1InitArgs in exactly the same way as JDK 1.1.
LINKAGE:
Exported from the native library that implements the Java virtual machine.
PARAMETERS:
p_vm: pointer to the location where the resulting VM structure will be placed.
p_env: pointer to the location where the JNI interface pointer for the main thread will be placed.
vm_args: Java VM initialization arguments.
RETURNS:
Returns “0” on success; returns a negative number on failure.
DestroyJavaVM
jint DestroyJavaVM(JavaVM *vm);
Unloads a Java VM and reclaims its resources. Only the main thread can unload the VM. The system waits until the main thread is only remaining user thread before it destroys the VM.
The support for DestroyJavaVM was not complete in 1.1. Only the main thread may call DestroyJavaVM. In the JDK, any thread, whether attached or not, can call this function. If the current thread is attached, the VM waits until the current thread is the only user-level Java thread. If the current thread is not attached, the VM attaches the current thread and then waits until the current thread is the only user-level thread. The JDK still does not support VM unloading, however. DestroyJavaVM always returns an error code.
LINKAGE:
Index 3 in the JavaVM interface function table.
PARAMETERS:
vm: the Java VM that will be destroyed.
RETURNS:
Returns “0” on success; returns a negative number on failure.
JDK 1.1.2 does not support unloading the VM.
AttachCurrentThread
jint AttachCurrentThread(JavaVM *vm, JNIEnv **p_env, void *thr_args);
Attaches the current thread to a Java VM. Returns a JNI interface pointer in the JNIEnv argument.
Trying to attach a thread that is already attached is a no-op.
A native thread cannot be attached simultaneously to two Java VMs.
When a thread is attached to the VM, the context class loader is the bootstrap loader.
LINKAGE:
Index 4 in the JavaVM interface function table.
PARAMETERS:
vm: the VM to which the current thread will be attached.
p_env: pointer to the location where the JNI interface pointer of the current thread will be placed.
thr_args: VM-specific thread attachment arguments.
In JDK 1.1, the second argument to AttachCurrentThread is always a pointer to JNIEnv. The third argument to AttachCurrentThread was reserved, and should be set to NULL.
In the JDK, you pass NULL as the third argument for 1.1 behavior, or pass a pointer to the following structure to specify additional information:
typedef struct JavaVMAttachArgs {
jint version; /* must be JNI_VERSION_1_2 */
char *name; /* the name of the thread as a modified UTF-8 string, or NULL */
jobject group; /* global ref of a ThreadGroup object, or NULL */
} JavaVMAttachArgs
RETURNS:
Returns “0” on success; returns a negative number on failure.
http://docs.oracle.com/javase/1. ... pec/invocation.html
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发表于 2012-12-18 21:50
