Java源码之深入解析Runnable和Thread的区别

我们都知道创建一个线程有两种方式一是继承Thread,一是实现Runnable接口。
那么java面试基本必问的一个问题就是这两种方式有什么区别。
其实区别有很多，简单讲Thread类本身也是实现了Runnable接口的。
所以继承了Thread类的线程类 都拥有实现Runnable接口的基本特性，但是实现了Runnable接口的线程类却不一定有Thread类的一些特性。

看看源码吧更清晰一点：

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

    private char        name[];
    private int         priority;
    private Thread      threadQ;
    private long        eetop;

    /* Whether or not to single_step this thread. */
    private boolean     single_step;

    /* Whether or not the thread is a daemon thread. */
    private boolean     daemon = false;

    /* JVM state */
    private boolean     stillborn = false;

    /* What will be run. */
    private Runnable target;

    /* The group of this thread */
    private ThreadGroup group;

    /* The context ClassLoader for this thread */
    private ClassLoader contextClassLoader;

    /* The inherited AccessControlContext of this thread */
    private AccessControlContext inheritedAccessControlContext;

    /* For autonumbering anonymous threads. */
    private static int threadInitNumber;
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

    /*
     * The requested stack size for this thread, or 0 if the creator did
     * not specify a stack size.  It is up to the VM to do whatever it
     * likes with this number; some VMs will ignore it.
     */
    private long stackSize;

    /*
     * JVM-private state that persists after native thread termination.
     */
    private long nativeParkEventPointer;

    /*
     * Thread ID
     */
    private long tid;

    /* For generating thread ID */
    private static long threadSeqNumber;

    /* Java thread status for tools,
     * initialized to indicate thread 'not yet started'
     */

    private volatile int threadStatus = 0;


    private static synchronized long nextThreadID() {
        return ++threadSeqNumber;
    }

    /**
     * The argument supplied to the current call to
     * java.util.concurrent.locks.LockSupport.park.
     * Set by (private) java.util.concurrent.locks.LockSupport.setBlocker
     * Accessed using java.util.concurrent.locks.LockSupport.getBlocker
     */
    volatile Object parkBlocker;

    /* The object in which this thread is blocked in an interruptible I/O
     * operation, if any.  The blocker's interrupt method should be invoked
     * after setting this thread's interrupt status.
     */
    private volatile Interruptible blocker;
    private final Object blockerLock = new Object();

贴了一小段，可以看到很清楚Thread类实现了Runnable接口。

而Runnable接口的定义则很简单：

 * Copyright (c) 1994, 2005, Oracle and/or its affiliates. All rights reserved.

package java.lang;

/**
 * The Runnable interface should be implemented by any
 * class whose instances are intended to be executed by a thread. The
 * class must define a method of no arguments called run.
 * 

 * This interface is designed to provide a common protocol for objects that
 * wish to execute code while they are active. For example,
 * Runnable is implemented by class Thread.
 * Being active simply means that a thread has been started and has not
 * yet been stopped.
 * 

 * In addition, Runnable provides the means for a class to be
 * active while not subclassing Thread. A class that implements
 * Runnable can run without subclassing Thread
 * by instantiating a Thread instance and passing itself in
 * as the target.  In most cases, the Runnable interface should
 * be used if you are only planning to override the run()
 * method and no other Thread methods.
 * This is important because classes should not be subclassed
 * unless the programmer intends on modifying or enhancing the fundamental
 * behavior of the class.
 *
 * @author  Arthur van Hoff
 * @see     java.lang.Thread
 * @see     java.util.concurrent.Callable
 * @since   JDK1.0
 */
public
interface Runnable {
    /**
     * When an object implementing interface Runnable is used
     * to create a thread, starting the thread causes the object's
     * run method to be called in that separately executing
     * thread.
     * 

     * The general contract of the method run is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

可以看到 如果我们编写了一个类，如果是继承的Thread类，可以重写run方法之后，实例化一个对象之后可以直接start,如果是实现了Runnable接口的类则实例化之后只能执行run方法，必须通过new Thread(Runnable runable)方法将线程类转化为Thread类 才可以start

这里讲述一下 start()和run()方法的区别，run是直接执行run方法的内容，start方法我们来看一下源码：

 /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the run method of this thread.
     * 

     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * start method) and the other thread (which executes its
     * run method).
     * 

     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    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 */
            }
        }
    }

    private native void start0();

注意此处调用的是start0（）。并且这个这个方法用了native关键字，该关键字表示调用本地操作系统的函数。因为多线程的实现需要本地操作系统的支持。

当然Thread类中也有对Runnable接口的run方法的实现：

    /**
     * If this thread was constructed using a separate
     * Runnable run object, then that
     * Runnable object's run method is called;
     * otherwise, this method does nothing and returns.
     * 

     * Subclasses of Thread should override this method.
     *
     * @see     #start()
     * @see     #stop()
     * @see     #Thread(ThreadGroup, Runnable, String)
     */
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }

因此 run方法只是一次执行并没有开启新的线程，start是系统级别开启一个线程然后在新的线程里面执行run方法。

此外 我们可以看到Thread类中内置了很对变量和方法，这都是Runable接口所没有的，其中比较关键的有：

    private char        name[];
     private int         priority;
    private Thread      threadQ;
     private ThreadGroup group;
      private static int threadInitNumber;
        private long tid;

    /* For generating thread ID */
    private static long threadSeqNumber;

    /* Java thread status for tools,
     * initialized to indicate thread 'not yet started'
     */

    private volatile int threadStatus = 0;