废话分析jvm
jvm是java虚拟机，这个还是要废话一下，语言这东西，翻译，执行，库基本上是必须的java的翻译不是编译，而是解释，热点的是编译？靠，大哥，佛家有云，空即是色，那话都没法说了，java是解释，一句一句的解释，碰到一段熟的，就翻译一下，这个从本质上说java还是整体解释，局部编译而已。
翻译执行也在jvm里，也就是我们安装后的一个dll，jvm.dll,还有好几种，什么客户端，服务器端
java的库呢，那就是我们经常说的jdk了，也就是sun帮我写了很多通用的东西，免得我们再写，他们专干这事的。我们也可以干，不过人家专而已，不要随便乱抢饭碗，得有实力。哈哈
所以java=jdk+jvm 或者什么jdk啊，jre啊，都可以，反正晓得是怎么回事就ok。
jvm是一个进程，是os的一个进程。这里要说的就是jvm有什么，jvm啥都没有
有一个翻译java代码成字节码的功能，有一个解释java字节码的功能，有一个内存管理的功能，没了。仅此而已，gc，那就是内存管理啊。线程？java没有线程，真正干活的是os的线程，网络？java也没有网络，干活的也是os。当然jvm干的也不是垃圾活，毕竟os干的有限。文件？java也没有。干了点活，然后交给os了。jni就是这个东东。其实一句话，jvm用c++写的，你想c++能干什么，java就能干什么，c++写的东西（除os外）java就可以干。关于这个，不必争论，见仁见智，只要不是空即是色，都还ok。最可恨的就是空即是色。啥都不是，啥都是，我快崩溃了。
下面就分析一个简单的hello world，运行java 的时候怎么执行的，一颗豌豆开始旅行了
哈哈
首先javac的就不分析了，毫无意思，编译的东西，将java源代码翻译成class字节码，以后会分析jvm如何执行字节码的，头都大了，又多又杂，不是人干的。
甭管它啥玩意，jvm是个dll，也就是一大堆函数，我们得找入口，入口就是main，毫无疑问
java.exe这个就是有main的，在java.c中我们找到了，这里只分析一下windowns的，也就是winmain，看
return JLI_Launch(margc, margv,
sizeof(const_jargs) / sizeof(char *), const_jargs,
sizeof(const_appclasspath) / sizeof(char *), const_appclasspath,
FULL_VERSION,
DOT_VERSION,
(const_progname != NULL) ? const_progname : *margv,
(const_launcher != NULL) ? const_launcher : *margv,
(const_jargs != NULL) ? JNI_TRUE : JNI_FALSE,
const_cpwildcard, const_javaw, const_ergo_class);
这个大家伙，winmain做了一些垃圾活后就交给这个函数了，
我们顺藤摸瓜。这个函数不用说了，肯定是首先初始化，分析参数，你想想，你用vc些个main，能干啥？不就是初始化，分析输入参数嘛 LoadJavaVM CreateExecutionEnvironment InitLauncher ParseArguments 一大堆，热火朝天啊。
初始化以后再说，我们分析大头，看这个 return ContinueInNewThread(&ifn, argc, argv, jarfile, classname, ret);
这个就发上调用ContinueInNewThread0 ，一般在函数后面带有0，1，2等的就是真正干活的，我们跟进去
int
ContinueInNewThread0(int (JNICALL *continuation)(void *), jlong stack_size, void * args) {
int rslt = 0;
unsigned thread_id;
#ifndef STACK_SIZE_PARAM_IS_A_RESERVATION
#define STACK_SIZE_PARAM_IS_A_RESERVATION(0x10000)
#endif
/*
* STACK_SIZE_PARAM_IS_A_RESERVATION is what we want, but it's not
* supported on older version of Windows. Try first with the flag; and
* if that fails try again without the flag. See MSDN document or HotSpot
* source (os_win32.cpp) for details.
*/
HANDLE thread_handle =
(HANDLE)_beginthreadex(NULL,
(unsigned)stack_size,
continuation,
args,
STACK_SIZE_PARAM_IS_A_RESERVATION,
&thread_id);
if (thread_handle == NULL) {
thread_handle =
(HANDLE)_beginthreadex(NULL,
(unsigned)stack_size,
continuation,
args,
0,
&thread_id);
}
if (thread_handle) {
WaitForSingleObject(thread_handle, INFINITE);
GetExitCodeThread(thread_handle, &rslt);
CloseHandle(thread_handle);
} else {
rslt = continuation(args);
}
return rslt;
}
看到没有，线程 windowns的线程调用 beginthreadex，调用哪个函数呢？ javaMain
我们看看
int JNICALL
JavaMain(void * _args)
{
JavaMainArgs *args = (JavaMainArgs *)_args;
int argc = args->argc;
char **argv = args->argv;
char *jarfile = args->jarfile;
char *classname = args->classname;
InvocationFunctions ifn = args->ifn;
JavaVM *vm = 0;
JNIEnv *env = 0;
jstring mainClassName;
jclass mainClass;
jmethodID mainID;
jobjectArray mainArgs;
int ret = 0;
jlong start, end;
/* Initialize the virtual machine */
start = CounterGet();
if (!InitializeJVM(&vm, &env, &ifn)) {
JLI_ReportErrorMessage(JVM_ERROR1);
exit(1);
}
if (printVersion || showVersion) {
PrintJavaVersion(env, showVersion);
CHECK_EXCEPTION_LEAVE(0);
if (printVersion) {
ret = 0;
goto leave;
}
}
/* If the user specified neither a class name nor a JAR file */
if (printXUsage || printUsage || (jarfile == 0 && classname == 0)) {
PrintUsage(env, printXUsage);
CHECK_EXCEPTION_LEAVE(1);
goto leave;
}
FreeKnownVMs();/* after last possible PrintUsage() */
if (JLI_IsTraceLauncher()) {
end = CounterGet();
JLI_TraceLauncher("%ld micro seconds to InitializeJVM\n",
(long)(jint)Counter2Micros(end-start));
}
/* At this stage, argc/argv have the applications' arguments */
if (JLI_IsTraceLauncher()){
int i;
printf("Main-Class is '%s'\n", classname ? classname : "");
printf("Apps' argc is %d\n", argc);
for (i=0; i < argc; i++) {
printf(" argv[%2d] = '%s'\n", i, argv);
}
}
ret = 1;
/*
* Get the application's main class.
*
* See bugid 5030265.The Main-Class name has already been parsed
* from the manifest, but not parsed properly for UTF-8 support.
* Hence the code here ignores the value previously extracted and
* uses the pre-existing code to reextract the value.This is
* possibly an end of release cycle expedient.However, it has
* also been discovered that passing some character sets through
* the environment has "strange" behavior on some variants of
* Windows.Hence, maybe the manifest parsing code local to the
* launcher should never be enhanced.
*
* Hence, future work should either:
* 1) Correct the local parsing code and verify that the
* Main-Class attribute gets properly passed through
* all environments,
* 2) Remove the vestages of maintaining main_class through
* the environment (and remove these comments).
*/
if (jarfile != 0) {
mainClass = LoadMainClass(env, JNI_TRUE, jarfile);
} else {
mainClass = LoadMainClass(env, JNI_FALSE, classname);
}
CHECK_EXCEPTION_NULL_LEAVE(mainClass);
/*
* The LoadMainClass not only loads the main class, it will also ensure
* that the main method's signature is correct, therefore further checking
* is not required. The main method is invoked here so that extraneous java
* stacks are not in the application stack trace.
*/
mainID = (*env)->GetStaticMethodID(env, mainClass, "main",
"([Ljava/lang/String;)V");
CHECK_EXCEPTION_NULL_LEAVE(mainID);
/* Build argument array */
mainArgs = NewPlatformStringArray(env, argv, argc);
CHECK_EXCEPTION_NULL_LEAVE(mainArgs);
/* Invoke main method. */
(*env)->CallStaticVoidMethod(env, mainClass, mainID, mainArgs);
/*
* The launcher's exit code (in the absence of calls to
* System.exit) will be non-zero if main threw an exception.
*/
ret = (*env)->ExceptionOccurred(env) == NULL ? 0 : 1;
leave:
/*
* Always detach the main thread so that it appears to have ended when
* the application's main method exits.This will invoke the
* uncaught exception handler machinery if main threw an
* exception.An uncaught exception handler cannot change the
* launcher's return code except by calling System.exit.
*/
if ((*vm)->DetachCurrentThread(vm) != 0) {
JLI_ReportErrorMessage(JVM_ERROR2);
ret = 1;
}
/*
* Wait for all non-daemon threads to end, then destroy the VM.
* This will actually create a trivial new Java waiter thread
* named "DestroyJavaVM", but this will be seen as a different
* thread from the one that executed main, even though they are
* the same C thread.This allows mainThread.join() and
* mainThread.isAlive() to work as expected.
*/
(*vm)->DestroyJavaVM(vm);
return ret;
}
好家伙，一大堆
别急，慢慢分析
InitializeJVM LoadMainClass(*env)->CallStaticVoidMethod(env, mainClass, mainID, mainArgs);
这三是干活的，2，8定理啊，汗
初始化jvm，该进程的jvm，一个进程一个jvm，加载helloworld类，调用helloworld的main方法，开始执行 ，要入正题了，七拐八拐，还算ok。
明天继续。。。
static jboolean
InitializeJVM(JavaVM **pvm, JNIEnv **penv, InvocationFunctions *ifn)
{
JavaVMInitArgs args;
jint r;
memset(&args, 0, sizeof(args));
args.version= JNI_VERSION_1_2;
args.nOptions = numOptions;
args.options= options;
args.ignoreUnrecognized = JNI_FALSE;
if (JLI_IsTraceLauncher()) {
int i = 0;
printf("JavaVM args:\n ");
printf("version 0x%08lx, ", (long)args.version);
printf("ignoreUnrecognized is %s, ",
args.ignoreUnrecognized ? "JNI_TRUE" : "JNI_FALSE");
printf("nOptions is %ld\n", (long)args.nOptions);
for (i = 0; i < numOptions; i++)
printf(" option[%2d] = '%s'\n",
i, args.options.optionString);
}
r = ifn->CreateJavaVM(pvm, (void **)penv, &args);
JLI_MemFree(options);
return r == JNI_OK;
}看这个 InitializeJVM 有个 r = ifn->CreateJavaVM(pvm, (void **)penv, &args);
就是创建jvm示例了。
好家伙，到jni了
HS_DTRACE_PROBE_DECL3(hotspot_jni, CreateJavaVM__entry, vm, penv, args);
DT_RETURN_MARK_DECL(CreateJavaVM, jint);
_JNI_IMPORT_OR_EXPORT_ jint JNICALL JNI_CreateJavaVM(JavaVM **vm, void **penv, void *args) {
HS_DTRACE_PROBE3(hotspot_jni, CreateJavaVM__entry, vm, penv, args);
jint result = JNI_ERR;
DT_RETURN_MARK(CreateJavaVM, jint, (const jint&)result);
// We're about to use Atomic::xchg for synchronization.Some Zero
// platforms use the GCC builtin __sync_lock_test_and_set for this,
// but __sync_lock_test_and_set is not guaranteed to do what we want
// on all architectures.So we check it works before relying on it.
#if defined(ZERO) && defined(ASSERT)
{
jint a = 0xcafebabe;
jint b = Atomic::xchg(0xdeadbeef, &a);
void *c = &a;
void *d = Atomic::xchg_ptr(&b, &c);
assert(a == (jint) 0xdeadbeef && b == (jint) 0xcafebabe, "Atomic::xchg() works");
assert(c == &b && d == &a, "Atomic::xchg_ptr() works");
}
#endif // ZERO && ASSERT
// At the moment it's only possible to have one Java VM,
// since some of the runtime state is in global variables.
// We cannot use our mutex locks here, since they only work on
// Threads. We do an atomic compare and exchange to ensure only
// one thread can call this method at a time
// We use Atomic::xchg rather than Atomic::add/dec since on some platforms
// the add/dec implementations are dependent on whether we are running
// on a multiprocessor, and at this stage of initialization the os::is_MP
// function used to determine this will always return false. Atomic::xchg
// does not have this problem.
if (Atomic::xchg(1, &vm_created) == 1) {
return JNI_ERR; // already created, or create attempt in progress
}
if (Atomic::xchg(0, &safe_to_recreate_vm) == 0) {
return JNI_ERR;// someone tried and failed and retry not allowed.
}
assert(vm_created == 1, "vm_created is true during the creation");
/**
* Certain errors during initialization are recoverable and do not
* prevent this method from being called again at a later time
* (perhaps with different arguments).However, at a certain
* point during initialization if an error occurs we cannot allow
* this function to be called again (or it will crash).In those
* situations, the 'canTryAgain' flag is set to false, which atomically
* sets safe_to_recreate_vm to 1, such that any new call to
* JNI_CreateJavaVM will immediately fail using the above logic.
*/
bool can_try_again = true;
result = Threads::create_vm((JavaVMInitArgs*) args, &can_try_again);
if (result == JNI_OK) {
JavaThread *thread = JavaThread::current();
/* thread is thread_in_vm here */
*vm = (JavaVM *)(&main_vm);
*(JNIEnv**)penv = thread->jni_environment();
// Tracks the time application was running before GC
RuntimeService::record_application_start();
// Notify JVMTI
if (JvmtiExport::should_post_thread_life()) {
JvmtiExport::post_thread_start(thread);
}
// Check if we should compile all classes on bootclasspath
NOT_PRODUCT(if (CompileTheWorld) ClassLoader::compile_the_world();)
// Since this is not a JVM_ENTRY we have to set the thread state manually before leaving.
ThreadStateTransition::transition_and_fence(thread, _thread_in_vm, _thread_in_native);
} else {
if (can_try_again) {
// reset safe_to_recreate_vm to 1 so that retrial would be possible
safe_to_recreate_vm = 1;
}
// Creation failed. We must reset vm_created
*vm = 0;
*(JNIEnv**)penv = 0;
// reset vm_created last to avoid race condition. Use OrderAccess to
// control both compiler and architectural-based reordering.
OrderAccess::release_store(&vm_created, 0);
}
return result;
}
Threads::create_vm((JavaVMInitArgs*) args, &can_try_again);
看这个，这个是不折不扣的jni也就是jvm的函数了，开始都是壳子而已。脱壳不容易啊
页:
[1]