JVM如何创建线程

java
java, jvm 
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Thread t = new Thread(new Runnable() {
    @Override
    public void run() {
        
    }
});
t.start();

上面这段代码在JVM中是如何实现的呢？

  1. 我们需要熟悉JNI

image

  1. 打开Thread.java

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    public
    class Thread implements Runnable {
        /* Make sure registerNatives is the first thing <clinit> does. */
        private static native void registerNatives();
        static {
            registerNatives();
        }
        // ...
    }

了解过JNI，那么我们知道Thread.java一定会对应一个c或c++文件，那么在JVM中Thread.java对应的C文件是openjdk-jdk8u-jdk8u\jdk\src\share\native\java\lang\Thread.c

打开Thread.c，发现这个文件很简单

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#include "jni.h"
#include "jvm.h"

#include "java_lang_Thread.h"

#define THD "Ljava/lang/Thread;"
#define OBJ "Ljava/lang/Object;"
#define STE "Ljava/lang/StackTraceElement;"
#define STR "Ljava/lang/String;"

#define ARRAY_LENGTH(a) (sizeof(a)/sizeof(a[0]))

static JNINativeMethod methods[] = {
    {"start0",           "()V",        (void *)&JVM_StartThread},
    {"stop0",            "(" OBJ ")V", (void *)&JVM_StopThread},
    {"isAlive",          "()Z",        (void *)&JVM_IsThreadAlive},
    {"suspend0",         "()V",        (void *)&JVM_SuspendThread},
    {"resume0",          "()V",        (void *)&JVM_ResumeThread},
    {"setPriority0",     "(I)V",       (void *)&JVM_SetThreadPriority},
    {"yield",            "()V",        (void *)&JVM_Yield},
    {"sleep",            "(J)V",       (void *)&JVM_Sleep},
    {"currentThread",    "()" THD,     (void *)&JVM_CurrentThread},
    {"countStackFrames", "()I",        (void *)&JVM_CountStackFrames},
    {"interrupt0",       "()V",        (void *)&JVM_Interrupt},
    {"isInterrupted",    "(Z)Z",       (void *)&JVM_IsInterrupted},
    {"holdsLock",        "(" OBJ ")Z", (void *)&JVM_HoldsLock},
    {"getThreads",        "()[" THD,   (void *)&JVM_GetAllThreads},
    {"dumpThreads",      "([" THD ")[[" STE, (void *)&JVM_DumpThreads},
    {"setNativeName",    "(" STR ")V", (void *)&JVM_SetNativeThreadName},
};

#undef THD
#undef OBJ
#undef STE
#undef STR

JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}

我们可以看到methods定义了所有的native方法。Java_java_lang_Thread_registerNatives恰好对应的是java.lang.Thread中的registerNatives()方法。

当执行t.start()的时候，发生了什么呢？

我们知道在执行t.start()的时候，jdk代码中会去执行start0的native方法那么对应的就是JVM_StartThread。

我们找到JVM_StartThread

openjdk-jdk8u-jdk8u\hotspot\src\share\vm\prims\jvm.h

openjdk-jdk8u-jdk8u\hotspot\src\share\vm\prims\jvm.cpp

在C/C++中.h .hpp文件为头文件，可以理解为啥呢，Java中的接口文件（类似哈，请不要较真）

.c .cpp为具体的实现文件。

在jvm.cpp找到JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))创建线程的入口。

image

其中第6步的new JavaThread在openjdk-jdk8u-jdk8u\hotspot\src\share\vm\runtime\thread.cpp

JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz)

在调用pthread_create的时候，这里涉及到具体的操作系统

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linux:
openjdk-jdk8u-jdk8u\hotspot\src\os\linux\vm\os_linux.cpp
windows:
openjdk-jdk8u-jdk8u\hotspot\src\os\windows\vm\os_windows.cpp
bsd:
openjdk-jdk8u-jdk8u\hotspot\src\os\bsd\vm\os_bsd.cpp
aix:
openjdk-jdk8u-jdk8u\hotspot\src\os\aix\vm\os_aix.cpp
solaris:
openjdk-jdk8u-jdk8u\hotspot\src\os\solaris\vm\os_solaris.cpp
本文由 xxzkid | 我叫王也道长创作，采用 CC BY 3.0 CN协议 进行许可。 可自由转载、引用，但需署名作者且注明文章出处。
本文标题：JVM如何创建线程
本文链接：https://xxzkid.github.io/2020/jvm-create-thread/