Java/JRE/JVM jni实现原理

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编程语言－JAVA/Scala/Clojure/Grovvy

Java中有种特殊的方法，叫native方法，也叫本机方法，这种方法和其他的方法不同在于通过native修饰，并且在定义native方法时不需要实现，其实现交由本机完成。参考https://lobin.iteye.com/blog/629414中的例子。

Java JDK中也有大量方法被定义为native方法。

Java JDK中的native方法由JVM实现。JVM在实现这些native方法时比较特殊，这些native实现一部分通过宏JVM_ENTRY，JVM_END来实现，另一部分在\jdk\src\share\native目录下。

通过宏JVM_ENTRY，JVM_END实现方式大概是这样的：

JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))
... ...
JVM_END

在看这部分代码时，经常看到THREAD，thread这两个线程相关的变量。这需要展开JVM_ENTRY宏才能看到他们的定义。

... ...
JavaThread* thread=JavaThread::thread_from_jni_environment(env); 
... ...
Thread* THREAD = thread;                                           
/* begin of body */

在编写java程序时，也可以定义native的java方法。

在开发Java JNI的时候，我们需要使用javah工具为native方法生成本机接口，javah会生成一个C/C++头文

件，头文件中包含没有native方法对应的本机方法函数的声明。

生成本机接口如下：

>javah -jni -d . JniNewTest

以下是生成的本机接口头文件样例：

/* DO NOT EDIT THIS FILE - it is machine generated */

#include <jni.h>

/* Header for class JniNewTest */

#ifndef _Included_JniNewTest

#define _Included_JniNewTest

#ifdef __cplusplus

extern "C" {

#endif

* Class: JniNewTest

* Method: testMethod

* Signature: ()V

JNIEXPORT void JNICALL Java_JniNewTest_testMethod

(JNIEnv *, jobject);

#ifdef __cplusplus

}

#endif

然后我们会实现这些本机方法函数，编译后链接生成动态链接库。最后在Java中通过loadLibrary加载动态库，这样Java的native方法就可以使用了。在使用之前要确保调用loadLibrary加载动态库。

System.loadLibrary("libjnitest");

可以看到生成的native本机接口的头文件中的函数定义，至少有两个参数，第一个是一个JNIEnv *的指针类型的参数，它表示Java Runtime运行时环境，它提供了运行时环境相关的接口Java Native Interface，或者Java Runtime Interface (JRI)，由Netscape定义。在C语言环境下，它是一个struct JNINativeInterface_ **的指针的指针结构体，在C++语言环境下，则是一个struct JNIEnv_*的指针结构体。第二个是几个native句柄。如果native方法是非static方法，该句柄是一个jobject类型的句柄，表示对应java对象的native句柄。如果是static方法，该句柄是一个jclass类型的句柄，表示对应java类的native句柄。

关于Java Runtime运行时环境提供的运行时环境相关的接口Java Native Interface，或者Java Runtime Interface (JRI)，参考另一篇文章：Java JNI: Java 本机接口, https://lobin.iteye.com/blog/2437928

那么这个运行时环境是怎么初始化的？

在hotspot\src\share\vm\prims\jni.cpp中定义了一个jni_NativeInterface变量，并对结构体中的成员字段进行了初始化。

// Structure containing all jni functions

struct JNINativeInterface_ jni_NativeInterface = {

NULL,

jni_GetVersion,

jni_DefineClass,

jni_FindClass,

jni_FromReflectedMethod,

jni_FromReflectedField,

jni_ToReflectedMethod,

jni_GetSuperclass,

jni_IsAssignableFrom,

jni_ToReflectedField,

jni_Throw,

jni_ThrowNew,

jni_ExceptionOccurred,

jni_ExceptionDescribe,

jni_ExceptionClear,

jni_FatalError,

jni_PushLocalFrame,

jni_PopLocalFrame,

jni_NewGlobalRef,

jni_DeleteGlobalRef,

jni_DeleteLocalRef,

jni_IsSameObject,

jni_NewLocalRef,

jni_EnsureLocalCapacity,

jni_AllocObject,

jni_NewObject,

jni_NewObjectV,

jni_NewObjectA,

jni_GetObjectClass,

jni_IsInstanceOf,

jni_GetMethodID,

jni_CallObjectMethod,

jni_CallObjectMethodV,

jni_CallObjectMethodA,

jni_CallBooleanMethod,

jni_CallBooleanMethodV,

jni_CallBooleanMethodA,

jni_CallByteMethod,

jni_CallByteMethodV,

jni_CallByteMethodA,

jni_CallCharMethod,

jni_CallCharMethodV,

jni_CallCharMethodA,

jni_CallShortMethod,

jni_CallShortMethodV,

jni_CallShortMethodA,

jni_CallIntMethod,

jni_CallIntMethodV,

jni_CallIntMethodA,

jni_CallLongMethod,

jni_CallLongMethodV,

jni_CallLongMethodA,

jni_CallFloatMethod,

jni_CallFloatMethodV,

jni_CallFloatMethodA,

jni_CallDoubleMethod,

jni_CallDoubleMethodV,

jni_CallDoubleMethodA,

jni_CallVoidMethod,

jni_CallVoidMethodV,

jni_CallVoidMethodA,

jni_CallNonvirtualObjectMethod,

jni_CallNonvirtualObjectMethodV,

jni_CallNonvirtualObjectMethodA,

jni_CallNonvirtualBooleanMethod,

jni_CallNonvirtualBooleanMethodV,

jni_CallNonvirtualBooleanMethodA,

jni_CallNonvirtualByteMethod,

jni_CallNonvirtualByteMethodV,

jni_CallNonvirtualByteMethodA,

jni_CallNonvirtualCharMethod,

jni_CallNonvirtualCharMethodV,

jni_CallNonvirtualCharMethodA,

jni_CallNonvirtualShortMethod,

jni_CallNonvirtualShortMethodV,

jni_CallNonvirtualShortMethodA,

jni_CallNonvirtualIntMethod,

jni_CallNonvirtualIntMethodV,

jni_CallNonvirtualIntMethodA,

jni_CallNonvirtualLongMethod,

jni_CallNonvirtualLongMethodV,

jni_CallNonvirtualLongMethodA,

jni_CallNonvirtualFloatMethod,

jni_CallNonvirtualFloatMethodV,

jni_CallNonvirtualFloatMethodA,

jni_CallNonvirtualDoubleMethod,

jni_CallNonvirtualDoubleMethodV,

jni_CallNonvirtualDoubleMethodA,

jni_CallNonvirtualVoidMethod,

jni_CallNonvirtualVoidMethodV,

jni_CallNonvirtualVoidMethodA,

jni_GetFieldID,

jni_GetObjectField,

jni_GetBooleanField,

jni_GetByteField,

jni_GetCharField,

jni_GetShortField,

jni_GetIntField,

jni_GetLongField,

jni_GetFloatField,

jni_GetDoubleField,

jni_SetObjectField,

jni_SetBooleanField,

jni_SetByteField,

jni_SetCharField,

jni_SetShortField,

jni_SetIntField,

jni_SetLongField,

jni_SetFloatField,

jni_SetDoubleField,

jni_GetStaticMethodID,

jni_CallStaticObjectMethod,

jni_CallStaticObjectMethodV,

jni_CallStaticObjectMethodA,

jni_CallStaticBooleanMethod,

jni_CallStaticBooleanMethodV,

jni_CallStaticBooleanMethodA,

jni_CallStaticByteMethod,

jni_CallStaticByteMethodV,

jni_CallStaticByteMethodA,

jni_CallStaticCharMethod,

jni_CallStaticCharMethodV,

jni_CallStaticCharMethodA,

jni_CallStaticShortMethod,

jni_CallStaticShortMethodV,

jni_CallStaticShortMethodA,

jni_CallStaticIntMethod,

jni_CallStaticIntMethodV,

jni_CallStaticIntMethodA,

jni_CallStaticLongMethod,

jni_CallStaticLongMethodV,

jni_CallStaticLongMethodA,

jni_CallStaticFloatMethod,

jni_CallStaticFloatMethodV,

jni_CallStaticFloatMethodA,

jni_CallStaticDoubleMethod,

jni_CallStaticDoubleMethodV,

jni_CallStaticDoubleMethodA,

jni_CallStaticVoidMethod,

jni_CallStaticVoidMethodV,

jni_CallStaticVoidMethodA,

jni_GetStaticFieldID,

jni_GetStaticObjectField,

jni_GetStaticBooleanField,

jni_GetStaticByteField,

jni_GetStaticCharField,

jni_GetStaticShortField,

jni_GetStaticIntField,

jni_GetStaticLongField,

jni_GetStaticFloatField,

jni_GetStaticDoubleField,

jni_SetStaticObjectField,

jni_SetStaticBooleanField,

jni_SetStaticByteField,

jni_SetStaticCharField,

jni_SetStaticShortField,

jni_SetStaticIntField,

jni_SetStaticLongField,

jni_SetStaticFloatField,

jni_SetStaticDoubleField,

jni_NewString,

jni_GetStringLength,

jni_GetStringChars,

jni_ReleaseStringChars,

jni_NewStringUTF,

jni_GetStringUTFLength,

jni_GetStringUTFChars,

jni_ReleaseStringUTFChars,

jni_GetArrayLength,

jni_NewObjectArray,

jni_GetObjectArrayElement,

jni_SetObjectArrayElement,

jni_NewBooleanArray,

jni_NewByteArray,

jni_NewCharArray,

jni_NewShortArray,

jni_NewIntArray,

jni_NewLongArray,

jni_NewFloatArray,

jni_NewDoubleArray,

jni_GetBooleanArrayElements,

jni_GetByteArrayElements,

jni_GetCharArrayElements,

jni_GetShortArrayElements,

jni_GetIntArrayElements,

jni_GetLongArrayElements,

jni_GetFloatArrayElements,

jni_GetDoubleArrayElements,

jni_ReleaseBooleanArrayElements,

jni_ReleaseByteArrayElements,

jni_ReleaseCharArrayElements,

jni_ReleaseShortArrayElements,

jni_ReleaseIntArrayElements,

jni_ReleaseLongArrayElements,

jni_ReleaseFloatArrayElements,

jni_ReleaseDoubleArrayElements,

jni_GetBooleanArrayRegion,

jni_GetByteArrayRegion,

jni_GetCharArrayRegion,

jni_GetShortArrayRegion,

jni_GetIntArrayRegion,

jni_GetLongArrayRegion,

jni_GetFloatArrayRegion,

jni_GetDoubleArrayRegion,

jni_SetBooleanArrayRegion,

jni_SetByteArrayRegion,

jni_SetCharArrayRegion,

jni_SetShortArrayRegion,

jni_SetIntArrayRegion,

jni_SetLongArrayRegion,

jni_SetFloatArrayRegion,

jni_SetDoubleArrayRegion,

jni_RegisterNatives,

jni_UnregisterNatives,

jni_MonitorEnter,

jni_MonitorExit,

jni_GetJavaVM,

jni_GetStringRegion,

jni_GetStringUTFRegion,

jni_GetPrimitiveArrayCritical,

jni_ReleasePrimitiveArrayCritical,

jni_GetStringCritical,

jni_ReleaseStringCritical,

jni_NewWeakGlobalRef,

jni_DeleteWeakGlobalRef,

jni_ExceptionCheck,

jni_NewDirectByteBuffer,

jni_GetDirectBufferAddress,

jni_GetDirectBufferCapacity,

// New 1_6 features

jni_GetObjectRefType

};

同时定义了一个函数jni_functions，该函数通过对应的指针返回jni_NativeInterface变量。

// Returns the function structure

struct JNINativeInterface_* jni_functions() {

#ifndef JNICHECK_KERNEL

if (CheckJNICalls) return jni_functions_check();

#else // JNICHECK_KERNEL

if (CheckJNICalls) warning("-Xcheck:jni is not supported in kernel vm.");

#endif // JNICHECK_KERNEL

return &jni_NativeInterface;

}

在构造JavaThread的时候，会调用initialize方法进行初始化，initialize函数通过set_jni_functions函数初始化

JNIEnv （即struct JNIEnv_）的_jni_environment成员。

在InitializeJVM初始化Java VM（虚拟机）的时候，在调用InvocationFunctions的CreateJavaVM函数（这里

其实是个函数指针，指向JNI_CreateJavaVM函数）时，通过Threads::create_vm创建一个VM（虚拟机），

并进行VM初始化，还会为VM创建一个线程（vm线程）：

jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {

extern void JDK_Version_init();

// Check version

if (!is_supported_jni_version(args->version)) return JNI_EVERSION;

// Initialize the output stream module

ostream_init();

// Process java launcher properties.

Arguments::process_sun_java_launcher_properties(args);

// Initialize the os module before using TLS

os::init();

// Initialize system properties.

Arguments::init_system_properties();

// So that JDK version can be used as a discrimintor when parsing arguments

JDK_Version_init();

// Update/Initialize System properties after JDK version number is known

Arguments::init_version_specific_system_properties();

// Parse arguments

jint parse_result = Arguments::parse(args);

if (parse_result != JNI_OK) return parse_result;

if (PauseAtStartup) {

os::pause();

}

#ifndef USDT2

HS_DTRACE_PROBE(hotspot, vm__init__begin);

#else /* USDT2 */

HOTSPOT_VM_INIT_BEGIN();

#endif /* USDT2 */

// Record VM creation timing statistics

TraceVmCreationTime create_vm_timer;

create_vm_timer.start();

// Timing (must come after argument parsing)

TraceTime timer("Create VM", TraceStartupTime);

// Initialize the os module after parsing the args

jint os_init_2_result = os::init_2();

if (os_init_2_result != JNI_OK) return os_init_2_result;

// Initialize output stream logging

ostream_init_log();

// Convert -Xrun to -agentlib: if there is no JVM_OnLoad

// Must be before create_vm_init_agents()

if (Arguments::init_libraries_at_startup()) {

convert_vm_init_libraries_to_agents();

}

// Launch -agentlib/-agentpath and converted -Xrun agents

if (Arguments::init_agents_at_startup()) {

create_vm_init_agents();

}

// Initialize Threads state

_thread_list = NULL;

_number_of_threads = 0;

_number_of_non_daemon_threads = 0;

// Initialize TLS

ThreadLocalStorage::init();

// Initialize global data structures and create system classes in heap

vm_init_globals();

// Attach the main thread to this os thread

JavaThread* main_thread = new JavaThread();

main_thread->set_thread_state(_thread_in_vm);

// must do this before set_active_handles and initialize_thread_local_storage

// Note: on solaris initialize_thread_local_storage() will (indirectly)

// change the stack size recorded here to one based on the java thread

// stacksize. This adjusted size is what is used to figure the placement

// of the guard pages.

main_thread->record_stack_base_and_size();

main_thread->initialize_thread_local_storage();

main_thread->set_active_handles(JNIHandleBlock::allocate_block());

if (!main_thread->set_as_starting_thread()) {

vm_shutdown_during_initialization(

"Failed necessary internal allocation. Out of swap space");

delete main_thread;

*canTryAgain = false; // don't let caller call JNI_CreateJavaVM again

return JNI_ENOMEM;

}

// Enable guard page *after* os::create_main_thread(), otherwise it would

// crash Linux VM, see notes in os_linux.cpp.

main_thread->create_stack_guard_pages();

// Initialize Java-Level synchronization subsystem

ObjectMonitor::Initialize() ;

// Initialize global modules

jint status = init_globals();

if (status != JNI_OK) {

delete main_thread;

*canTryAgain = false; // don't let caller call JNI_CreateJavaVM again

return status;

}

// Should be done after the heap is fully created

main_thread->cache_global_variables();

HandleMark hm;

{ MutexLocker mu(Threads_lock);

Threads::add(main_thread);

}

// Any JVMTI raw monitors entered in onload will transition into

// real raw monitor. VM is setup enough here for raw monitor enter.

JvmtiExport::transition_pending_onload_raw_monitors();

if (VerifyBeforeGC &&

Universe::heap()->total_collections() >= VerifyGCStartAt) {

Universe::heap()->prepare_for_verify();

Universe::verify(); // make sure we're starting with a clean slate

}

// Create the VMThread

{ TraceTime timer("Start VMThread", TraceStartupTime);

VMThread::create();

Thread* vmthread = VMThread::vm_thread();

if (!os::create_thread(vmthread, os::vm_thread))

vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");

// Wait for the VM thread to become ready, and VMThread::run to initialize

// Monitors can have spurious returns, must always check another state flag

{

MutexLocker ml(Notify_lock);

os::start_thread(vmthread);

while (vmthread->active_handles() == NULL) {

Notify_lock->wait();

}

assert (Universe::is_fully_initialized(), "not initialized");

EXCEPTION_MARK;

// At this point, the Universe is initialized, but we have not executed

// any byte code. Now is a good time (the only time) to dump out the

// internal state of the JVM for sharing.

if (DumpSharedSpaces) {

Universe::heap()->preload_and_dump(CHECK_0);

ShouldNotReachHere();

}

// Always call even when there are not JVMTI environments yet, since environments

// may be attached late and JVMTI must track phases of VM execution

JvmtiExport::enter_start_phase();

// Notify JVMTI agents that VM has started (JNI is up) - nop if no agents.

JvmtiExport::post_vm_start();

{

TraceTime timer("Initialize java.lang classes", TraceStartupTime);

if (EagerXrunInit && Arguments::init_libraries_at_startup()) {

create_vm_init_libraries();

}

if (InitializeJavaLangString) {

initialize_class(vmSymbols::java_lang_String(), CHECK_0);

} else {

warning("java.lang.String not initialized");

}

if (AggressiveOpts) {

{

// Forcibly initialize java/util/HashMap and mutate the private

// static final "frontCacheEnabled" field before we start creating instances

#ifdef ASSERT

klassOop tmp_k = SystemDictionary::find(vmSymbols::java_util_HashMap(), Handle(), Handle(), CHECK_0);

assert(tmp_k == NULL, "java/util/HashMap should not be loaded yet");

#endif

klassOop k_o = SystemDictionary::resolve_or_null(vmSymbols::java_util_HashMap(), Handle(), Handle(), CHECK_0);

KlassHandle k = KlassHandle(THREAD, k_o);

guarantee(k.not_null(), "Must find java/util/HashMap");

instanceKlassHandle ik = instanceKlassHandle(THREAD, k());

ik->initialize(CHECK_0);

fieldDescriptor fd;

// Possible we might not find this field; if so, don't break

if (ik->find_local_field(vmSymbols::frontCacheEnabled_name(), vmSymbols::bool_signature(), &fd)) {

k()->java_mirror()->bool_field_put(fd.offset(), true);

}

if (UseStringCache) {

// Forcibly initialize java/lang/StringValue and mutate the private

// static final "stringCacheEnabled" field before we start creating instances

klassOop k_o = SystemDictionary::resolve_or_null(vmSymbols::java_lang_StringValue(), Handle(), Handle(), CHECK_0);

// Possible that StringValue isn't present: if so, silently don't break

if (k_o != NULL) {

KlassHandle k = KlassHandle(THREAD, k_o);

instanceKlassHandle ik = instanceKlassHandle(THREAD, k());

ik->initialize(CHECK_0);

fieldDescriptor fd;

// Possible we might not find this field: if so, silently don't break

if (ik->find_local_field(vmSymbols::stringCacheEnabled_name(), vmSymbols::bool_signature(), &fd)) {

k()->java_mirror()->bool_field_put(fd.offset(), true);

}

// Initialize java_lang.System (needed before creating the thread)

if (InitializeJavaLangSystem) {

initialize_class(vmSymbols::java_lang_System(), CHECK_0);

initialize_class(vmSymbols::java_lang_ThreadGroup(), CHECK_0);

Handle thread_group = create_initial_thread_group(CHECK_0);

Universe::set_main_thread_group(thread_group());

initialize_class(vmSymbols::java_lang_Thread(), CHECK_0);

oop thread_object = create_initial_thread(thread_group, main_thread, CHECK_0);

main_thread->set_threadObj(thread_object);

// Set thread status to running since main thread has

// been started and running.

java_lang_Thread::set_thread_status(thread_object,

java_lang_Thread::RUNNABLE);

// The VM preresolve methods to these classes. Make sure that get initialized

initialize_class(vmSymbols::java_lang_reflect_Method(), CHECK_0);

initialize_class(vmSymbols::java_lang_ref_Finalizer(), CHECK_0);

// The VM creates & returns objects of this class. Make sure it's initialized.

initialize_class(vmSymbols::java_lang_Class(), CHECK_0);

call_initializeSystemClass(CHECK_0);

} else {

warning("java.lang.System not initialized");

}

// an instance of OutOfMemory exception has been allocated earlier

if (InitializeJavaLangExceptionsErrors) {

initialize_class(vmSymbols::java_lang_OutOfMemoryError(), CHECK_0);

initialize_class(vmSymbols::java_lang_NullPointerException(), CHECK_0);

initialize_class(vmSymbols::java_lang_ClassCastException(), CHECK_0);

initialize_class(vmSymbols::java_lang_ArrayStoreException(), CHECK_0);

initialize_class(vmSymbols::java_lang_ArithmeticException(), CHECK_0);

initialize_class(vmSymbols::java_lang_StackOverflowError(), CHECK_0);

initialize_class(vmSymbols::java_lang_IllegalMonitorStateException(), CHECK_0);

initialize_class(vmSymbols::java_lang_IllegalArgumentException(), CHECK_0);

} else {

warning("java.lang.OutOfMemoryError has not been initialized");

warning("java.lang.NullPointerException has not been initialized");

warning("java.lang.ClassCastException has not been initialized");

warning("java.lang.ArrayStoreException has not been initialized");

warning("java.lang.ArithmeticException has not been initialized");

warning("java.lang.StackOverflowError has not been initialized");

warning("java.lang.IllegalArgumentException has not been initialized");

}

// See : bugid 4211085.

// Background : the static initializer of java.lang.Compiler tries to read

// property"java.compiler" and read & write property "java.vm.info".

// When a security manager is installed through the command line

// option "-Djava.security.manager", the above properties are not

// readable and the static initializer for java.lang.Compiler fails

// resulting in a NoClassDefFoundError. This can happen in any

// user code which calls methods in java.lang.Compiler.

// Hack : the hack is to pre-load and initialize this class, so that only

// system domains are on the stack when the properties are read.

// Currently even the AWT code has calls to methods in java.lang.Compiler.

// On the classic VM, java.lang.Compiler is loaded very early to load the JIT.

// Future Fix : the best fix is to grant everyone permissions to read "java.compiler" and

// read and write"java.vm.info" in the default policy file. See bugid 4211383

// Once that is done, we should remove this hack.

initialize_class(vmSymbols::java_lang_Compiler(), CHECK_0);

// More hackery - the static initializer of java.lang.Compiler adds the string "nojit" to

// the java.vm.info property if no jit gets loaded through java.lang.Compiler (the hotspot

// compiler does not get loaded through java.lang.Compiler). "java -version" with the

// hotspot vm says "nojit" all the time which is confusing. So, we reset it here.

// This should also be taken out as soon as 4211383 gets fixed.

reset_vm_info_property(CHECK_0);

quicken_jni_functions();

// Must be run after init_ft which initializes ft_enabled

if (TRACE_INITIALIZE() != JNI_OK) {

vm_exit_during_initialization("Failed to initialize tracing backend");

}

// Set flag that basic initialization has completed. Used by exceptions and various

// debug stuff, that does not work until all basic classes have been initialized.

set_init_completed();

#ifndef USDT2

HS_DTRACE_PROBE(hotspot, vm__init__end);

#else /* USDT2 */

HOTSPOT_VM_INIT_END();

#endif /* USDT2 */

// record VM initialization completion time

Management::record_vm_init_completed();

// Compute system loader. Note that this has to occur after set_init_completed, since

// valid exceptions may be thrown in the process.

// Note that we do not use CHECK_0 here since we are inside an EXCEPTION_MARK and

// set_init_completed has just been called, causing exceptions not to be shortcut

// anymore. We call vm_exit_during_initialization directly instead.

SystemDictionary::compute_java_system_loader(THREAD);

if (HAS_PENDING_EXCEPTION) {

vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));

}

#ifndef SERIALGC

// Support for ConcurrentMarkSweep. This should be cleaned up

// and better encapsulated. The ugly nested if test would go away

// once things are properly refactored. XXX YSR

if (UseConcMarkSweepGC || UseG1GC) {

if (UseConcMarkSweepGC) {

ConcurrentMarkSweepThread::makeSurrogateLockerThread(THREAD);

} else {

ConcurrentMarkThread::makeSurrogateLockerThread(THREAD);

}

if (HAS_PENDING_EXCEPTION) {

vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));

}

#endif // SERIALGC

// Always call even when there are not JVMTI environments yet, since environments

// may be attached late and JVMTI must track phases of VM execution

JvmtiExport::enter_live_phase();

// Signal Dispatcher needs to be started before VMInit event is posted

os::signal_init();

// Start Attach Listener if +StartAttachListener or it can't be started lazily

if (!DisableAttachMechanism) {

if (StartAttachListener || AttachListener::init_at_startup()) {

AttachListener::init();

}

// Launch -Xrun agents

// Must be done in the JVMTI live phase so that for backward compatibility the JDWP

// back-end can launch with -Xdebug -Xrunjdwp.

if (!EagerXrunInit && Arguments::init_libraries_at_startup()) {

create_vm_init_libraries();

}

if (!TRACE_START()) {

vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));

}

// Notify JVMTI agents that VM initialization is complete - nop if no agents.

JvmtiExport::post_vm_initialized();

if (CleanChunkPoolAsync) {

Chunk::start_chunk_pool_cleaner_task();

}

// initialize compiler(s)

CompileBroker::compilation_init();

Management::initialize(THREAD);

if (HAS_PENDING_EXCEPTION) {

// management agent fails to start possibly due to

// configuration problem and is responsible for printing

// stack trace if appropriate. Simply exit VM.

vm_exit(1);

}

if (Arguments::has_profile()) FlatProfiler::engage(main_thread, true);

if (Arguments::has_alloc_profile()) AllocationProfiler::engage();

if (MemProfiling) MemProfiler::engage();

StatSampler::engage();

if (CheckJNICalls) JniPeriodicChecker::engage();

BiasedLocking::init();

if (JDK_Version::current().post_vm_init_hook_enabled()) {

call_postVMInitHook(THREAD);

// The Java side of PostVMInitHook.run must deal with all

// exceptions and provide means of diagnosis.

if (HAS_PENDING_EXCEPTION) {

CLEAR_PENDING_EXCEPTION;

}

// Start up the WatcherThread if there are any periodic tasks

// NOTE: All PeriodicTasks should be registered by now. If they

// aren't, late joiners might appear to start slowly (we might

// take a while to process their first tick).

if (PeriodicTask::num_tasks() > 0) {

WatcherThread::start();

}

// Give os specific code one last chance to start

os::init_3();

create_vm_timer.end();

return JNI_OK;

}

为VM创建一个线程（vm线程）的代码：

// Create the VMThread

{ TraceTime timer("Start VMThread", TraceStartupTime);

VMThread::create();

Thread* vmthread = VMThread::vm_thread();

if (!os::create_thread(vmthread, os::vm_thread))

vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");

// Wait for the VM thread to become ready, and VMThread::run to initialize

// Monitors can have spurious returns, must always check another state flag

{

MutexLocker ml(Notify_lock);

os::start_thread(vmthread);

while (vmthread->active_handles() == NULL) {

Notify_lock->wait();

}

同时还会创建一个主线程，代码如下：

// Attach the main thread to this os thread

JavaThread* main_thread = new JavaThread();

main_thread->set_thread_state(_thread_in_vm);

// must do this before set_active_handles and initialize_thread_local_storage

// Note: on solaris initialize_thread_local_storage() will (indirectly)

// change the stack size recorded here to one based on the java thread

// stacksize. This adjusted size is what is used to figure the placement

// of the guard pages.

main_thread->record_stack_base_and_size();

main_thread->initialize_thread_local_storage();

Java VM（虚拟机）创建后，从对应的主线程中得到这个运行时环境。

这个运行时环境就是这么来的。。。

以structJNINativeInterface_的FindClass为例

jclass (JNICALL *FindClass) (JNIEnv *env, const char *name);

在jni.cpp中可以看出对应的函数实现为jni_FindClass

jni_FindClass对应的实现：

jni_FindClass对应的实现

static bool first_time_FindClass = true;

#ifndef USDT2

DT_RETURN_MARK_DECL(FindClass, jclass);

#else /* USDT2 */

DT_RETURN_MARK_DECL(FindClass, jclass

, HOTSPOT_JNI_FINDCLASS_RETURN(_ret_ref));

#endif /* USDT2 */

JNI_ENTRY(jclass, jni_FindClass(JNIEnv *env, const char *name))

JNIWrapper("FindClass");

#ifndef USDT2

DTRACE_PROBE2(hotspot_jni, FindClass__entry, env, name);

#else /* USDT2 */

HOTSPOT_JNI_FINDCLASS_ENTRY(

env, (char *)name);

#endif /* USDT2 */

jclass result = NULL;

DT_RETURN_MARK(FindClass, jclass, (const jclass&)result);

// Remember if we are the first invocation of jni_FindClass

bool first_time = first_time_FindClass;

first_time_FindClass = false;

// Sanity check the name: it cannot be null or larger than the maximum size

// name we can fit in the constant pool.

if (name == NULL || (int)strlen(name) > Symbol::max_length()) {

THROW_MSG_0(vmSymbols::java_lang_NoClassDefFoundError(), name);

}

//%note jni_3

Handle loader;

Handle protection_domain;

// Find calling class

instanceKlassHandle k (THREAD, thread->security_get_caller_class(0));

if (k.not_null()) {

loader = Handle(THREAD, k->class_loader());

// Special handling to make sure JNI_OnLoad and JNI_OnUnload are executed

// in the correct class context.

if (loader.is_null() &&

k->name() == vmSymbols::java_lang_ClassLoader_NativeLibrary()) {

JavaValue result(T_OBJECT);

JavaCalls::call_static(&result, k,

vmSymbols::getFromClass_name(),

vmSymbols::void_class_signature(),

thread);

if (HAS_PENDING_EXCEPTION) {

Handle ex(thread, thread->pending_exception());

CLEAR_PENDING_EXCEPTION;

THROW_HANDLE_0(ex);

}

oop mirror = (oop) result.get_jobject();

loader = Handle(THREAD,

instanceKlass::cast(java_lang_Class::as_klassOop(mirror))->class_loader());

protection_domain = Handle(THREAD,

instanceKlass::cast(java_lang_Class::as_klassOop(mirror))->protection_domain());

}

} else {

// We call ClassLoader.getSystemClassLoader to obtain the system class loader.

loader = Handle(THREAD, SystemDictionary::java_system_loader());

}

TempNewSymbol sym = SymbolTable::new_symbol(name, CHECK_NULL);

result = find_class_from_class_loader(env, sym, true, loader,

protection_domain, true, thread);

if (TraceClassResolution && result != NULL) {

trace_class_resolution(java_lang_Class::as_klassOop(JNIHandles::resolve_non_null(result)));

}

// If we were the first invocation of jni_FindClass, we enable compilation again

// rather than just allowing invocation counter to overflow and decay.

// Controlled by flag DelayCompilationDuringStartup.

if (first_time && !CompileTheWorld)

CompilationPolicy::completed_vm_startup();

return result;

JNI_END

分解后

static bool first_time_FindClass = true;

#ifndef USDT2

#else /* USDT2 */

#endif /* USDT2 */

extern "C" {

jclass JNICALL jni_FindClass(JNIEnv *env, const char *name) {

JavaThread* thread=JavaThread::thread_from_jni_environment(env);

assert( !VerifyJNIEnvThread || (thread == Thread::current()), "JNIEnv is only valid in same thread");

ThreadInVMfromNative __tiv(thread);

debug_only(VMNativeEntryWrapper __vew;)

VM_ENTRY_BASE(jclass, jni_FindClass(JNIEnv *env, const char *name), thread)

WeakPreserveExceptionMark __wem(thread);

JNIWrapper("FindClass");

#ifndef USDT2

{;}

;

#else /* USDT2 */

;

#endif /* USDT2 */

jclass result = NULL;

// Remember if we are the first invocation of jni_FindClass

bool first_time = first_time_FindClass;

first_time_FindClass = false;

// Sanity check the name: it cannot be null or larger than the maximum size

// name we can fit in the constant pool.

if (name == NULL || (int)strlen(name) > Symbol::max_length()) {

{

Exceptions::_throw_msg(THREAD_AND_LOCATION, vmSymbols::java_lang_NoClassDefFoundError(), name);

return result;

}

;

}

//%note jni_3

Handle loader;

Handle protection_domain;

// Find calling class

instanceKlassHandle k (THREAD, thread->security_get_caller_class(0));

if (k.not_null()) {

loader = Handle(THREAD, k->class_loader());

// Special handling to make sure JNI_OnLoad and JNI_OnUnload are executed

// in the correct class context.

if (loader.is_null() &&

k->name() == vmSymbols::java_lang_ClassLoader_NativeLibrary()) {

JavaValue result(T_OBJECT);

JavaCalls::call_static(&result, k,

vmSymbols::getFromClass_name(),

vmSymbols::void_class_signature(),

thread);

if (HAS_PENDING_EXCEPTION) {

Handle ex(thread, thread->pending_exception());

CLEAR_PENDING_EXCEPTION;

THROW_HANDLE_0(ex);

}

oop mirror = (oop) result.get_jobject();

loader = Handle(THREAD,

instanceKlass::cast(java_lang_Class::as_klassOop(mirror))->class_loader());

protection_domain = Handle(THREAD,

instanceKlass::cast(java_lang_Class::as_klassOop(mirror))->protection_domain());

}

} else {

// We call ClassLoader.getSystemClassLoader to obtain the system class loader.

loader = Handle(THREAD, SystemDictionary::java_system_loader());

}

TempNewSymbol sym = SymbolTable::new_symbol(name, CHECK_NULL);

result = find_class_from_class_loader(env, sym, true, loader,

protection_domain, true, thread);

if (TraceClassResolution && result != NULL) {

trace_class_resolution(java_lang_Class::as_klassOop(JNIHandles::resolve_non_null(result)));

}

// If we were the first invocation of jni_FindClass, we enable compilation again

// rather than just allowing invocation counter to overflow and decay.

// Controlled by flag DelayCompilationDuringStartup.

if (first_time && !CompileTheWorld)

CompilationPolicy::completed_vm_startup();

return result;

}

那么动态链接库是怎么加载的？除了我们自行开发的动态链接库，Java JDK内部也有很多native方法，对应在Java运行时环境JRE中也有对应的动态库。

还有这些动态库是怎么注册的？

Java native方法是怎么和本机方法实现绑定的？

在调用Java native方法时是怎么lookup对应的实现的？

Java在运行时实时编译时，如果编译的方法是个native方法，调用NativeLookup::lookup查找native方法的实现。

这在CompileBroker::compile_method中会调用NativeLookup::lookup查找native方法的实现。参考hotspot\src\share\vm\compiler\compileBroker.cpp的compile_method方法实现。另外

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线程安全 | 关于ext文件系统

2016-07-18 18:59
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