Java線程Run和Start的區別

先上結論：run只是Thread裡面的一個普通方法，start是啟動線程的方法。何以見得呢？可以執行下麵的代碼看看run和start的區別：

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();
    }

}

　　執行結果：

　　由此可以看到子線程是由start來啟動的，裡面調用了run，所以列印出來的是子線程的name。

　　另外也可以從start方法的底層代碼看到，首先進入start方法裡面

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 */
            }
        }
    }

　　裡面調用了start0，繼續跟進

private native void start0();

　　哦，這裡就調了native方法了，再往下我們就看不到了。但是可以去openjdk的源碼裡面去看一下，我用的是jdk8。

　　去openjdk看源碼

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

　　這裡調用了JVM_StartThread，在jvm.h裡面

　　繼續進入

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

　　重點看最下麵的語句native_thread = new JavaThread(&thread_entry, sz)，好，new JavaThread的時候傳入了thread_entry，我們再去

看一下啊thread_entry是什麼：

　　在thread_entry這個函數裡面調用call_virtual方法，重點是它傳入的參數run_method_name，從這裡我們就知道了，噢，

原來start最終會在新線程裡面調用run方法。