一、簡介

在Java代碼當(dāng)中，當(dāng)我們需要開啟子線程去處理一些任務(wù)的時候，往往是調(diào)用Thread對象的start方法，這樣Thread實例中的Runnable對象的run方法就會在一個新的線程當(dāng)中執(zhí)行；

// 創(chuàng)建一個線程 Thread thread = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("working in " + Thread.currentThread().getName());} });// 在什么線程調(diào)用，run方法的任務(wù)就在什么線程執(zhí)行 thread.run();// 在一個新的線程中執(zhí)行run方法中的任務(wù) thread.start();

二、分析run()方法

如下代碼可見，當(dāng)我們直接調(diào)用run方法的時候，就相當(dāng)于實例直接調(diào)用一個普通的方法一樣，所以無論我們在什么線程中調(diào)用run方法，run方法中的業(yè)務(wù)就在什么線程執(zhí)行，并不會說啟動一個新的線程去執(zhí)行run方法中的任務(wù)；

// Thread.class中的run方法，target為Runnable實例 @Override public void run() {if (target != null) {target.run();} }

三、分析start()方法

如下我們直接來看Thread.class中的代碼；

public synchronized void start() {/*** This method is not invoked for the main method thread or "system"* group threads created/set up by the VM. Any new functionality added* to this method in the future may have to also be added to the VM.** A zero status value corresponds to state "NEW".*/if (threadStatus != 0)throw new IllegalThreadStateException();/* Notify the group that this thread is about to be started* so that it can be added to the group's list of threads* and the group's unstarted count can be decremented. */// 1. 將當(dāng)前線程實例添加到ThreadGroup對象group中group.add(this);boolean started = false;try {// 2. 調(diào)用native方法start0()start0();started = true;} finally {try {if (!started) {group.threadStartFailed(this);}} catch (Throwable ignore) {/* do nothing. If start0 threw a Throwable thenit will be passed up the call stack */}} }// native 方法 private native void start0();

看上面代碼的注釋1那里，我們來看下Thread.class代碼里面group是哪里來的

private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc,boolean inheritThreadLocals) {if (name == null) {throw new NullPointerException("name cannot be null");}this.name = name;Thread parent = currentThread();// 1. 從System類中獲取安全管理器SecurityManager security = System.getSecurityManager();if (g == null) {/* Determine if it's an applet or not *//* If there is a security manager, ask the security managerwhat to do. */if (security != null) {// 2. 從安全管理器中過去ThreadGroup對象g = security.getThreadGroup();}/* If the security doesn't have a strong opinion of the matteruse the parent thread group. */if (g == null) {g = parent.getThreadGroup();}}g.addUnstarted();this.group = g;// 省略一部分代碼··· }

從上面注釋1,2可以看出，Thread實例中的ThreadGroup實例g主要是從底層代碼中獲取的，即group.add(this)執(zhí)行之后，將當(dāng)前線程傳給了底層系統(tǒng)去管理

下面看下start0()方法，因為start0()方法為Native方法，看下是怎么實現(xiàn)的，如下為src/main/native/Thread.c的代碼

static JNINativeMethod methods[] = {// 從這里我們可以看到start0方法主要JVM_StartThread的一個方法引用{"start0", "(JZ)V", (void *)&JVM_StartThread},{"setPriority0", "(I)V", (void *)&JVM_SetThreadPriority},{"yield", "()V", (void *)&JVM_Yield},{"sleep", "(Ljava/lang/Object;J)V", (void *)&JVM_Sleep},{"currentThread", "()" THD, (void *)&JVM_CurrentThread},{"interrupt0", "()V", (void *)&JVM_Interrupt},{"isInterrupted", "(Z)Z", (void *)&JVM_IsInterrupted},{"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock},{"setNativeName", "(" STR ")V", (void *)&JVM_SetNativeThreadName}, };

下面一下JVM_StartThread的情況

// 如下為/openjdkjvm/OpenjdkJvm.cc中的代碼 JNIEXPORT void JVM_StartThread(JNIEnv* env, jobject jthread, jlong stack_size, jboolean daemon) {art::Thread::CreateNativeThread(env, jthread, stack_size, daemon == JNI_TRUE); }// 如下為/runtime/thread.cc中創(chuàng)建native線程的代碼 void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {···// 這里就不做展開了 }

總結(jié)：從上面代碼可以看出，Thread調(diào)用start的時候，將thread實例傳給虛擬機持有，然后調(diào)用native創(chuàng)建一個新的線程，由虛擬機進行調(diào)度。

水平有限，有誤請指正！

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