为什么使用start方法启动Java的Thread线程？

一、简介

在Java代码当中，当我们需要开启子线程去处理一些任务的时候，往往是调用Thread对象的start方法，这样Thread实例中的Runnable对象的run方法就会在一个新的线程当中执行；

// 创建一个线程
Thread thread = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("working in " + Thread.currentThread().getName());}
});// 在什么线程调用，run方法的任务就在什么线程执行
thread.run();// 在一个新的线程中执行run方法中的任务
thread.start();

二、分析run()方法

如下代码可见，当我们直接调用run方法的时候，就相当于实例直接调用一个普通的方法一样，所以无论我们在什么线程中调用run方法，run方法中的业务就在什么线程执行，并不会说启动一个新的线程去执行run方法中的任务；

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

三、分析start()方法

如下我们直接来看Thread.class中的代码；

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. 将当前线程实例添加到ThreadGroup对象group中group.add(this);boolean started = false;try {// 2. 调用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();

看上面代码的注释1那里，我们来看下Thread.class代码里面group是哪里来的

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. 从System类中获取安全管理器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. 从安全管理器中过去ThreadGroup对象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;// 省略一部分代码···
}

从上面注释1,2可以看出，Thread实例中的ThreadGroup实例g主要是从底层代码中获取的，即group.add(this)执行之后，将当前线程传给了底层系统去管理

下面看下start0()方法，因为start0()方法为Native方法，看下是怎么实现的，如下为src/main/native/Thread.c的代码

static JNINativeMethod methods[] = {// 从这里我们可以看到start0方法主要JVM_StartThread的一个方法引用{"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的情况

// 如下为/openjdkjvm/OpenjdkJvm.cc中的代码
JNIEXPORT void JVM_StartThread(JNIEnv* env, jobject jthread, jlong stack_size, jboolean daemon) {art::Thread::CreateNativeThread(env, jthread, stack_size, daemon == JNI_TRUE);
}// 如下为/runtime/thread.cc中创建native线程的代码
void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {···// 这里就不做展开了
}

总结：从上面代码可以看出，Thread调用start的时候，将thread实例传给虚拟机持有，然后调用native创建一个新的线程，由虚拟机进行调度。

水平有限，有误请指正！