Java线程Run和Start的区别

先上结论：run只是Thread里面的一个普通方法，start是启动线程的方法。何以见得呢？可以执行下面的代码看看run和start的区别：

package com.basic.thread;

/**
 * @author zhangxingrui
 * @create 2019-02-16 20:12
 **/
public class TestRunAndStart {

    private static void sayHello(){
        System.out.println("hello, world");
    }

    public static void main(String[] args) {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                sayHello();
                System.out.println("Current thread: " + Thread.currentThread().getName());
            }
        });

        thread.run();
        thread.start();
    }

}

　　执行结果：

　　由此可以看到子线程是由start来启动的，里面调用了run，所以打印出来的是子线程的name。

　　另外也可以从start方法的底层代码看到，首先进入start方法里面

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. */
        group.add(this);

        boolean started = false;
        try {
            start0();
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }

　　里面调用了start0，继续跟进

private native void start0();

　　哦，这里就调了native方法了，再往下我们就看不到了。但是可以去openjdk的源码里面去看一下，我用的是jdk8。

　　去openjdk看源码

　　https://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/f0b93fbd8cf8/src/share/native/java/lang/Thread.c

　　

　　这里调用了JVM_StartThread，在jvm.h里面

　　继续进入

　　https://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/76a9c9cf14f1/src/share/vm/prims/jvm.cpp

　　

　　重点看最下面的语句native_thread = new JavaThread(&thread_entry, sz)，好，new JavaThread的时候传入了thread_entry，我们再去

看一下啊thread_entry是什么：

　　

　　在thread_entry这个函数里面调用call_virtual方法，重点是它传入的参数run_method_name，从这里我们就知道了，噢，

原来start最终会在新线程里面调用run方法。