java Thread

一. Thread类与object类等其他类一样，刚开始就有一个

    private static native void registerNatives();
    static {
        registerNatives();
    }

这个本地方法是为了让这个class的所有本地方法（例如C）可以自定义名称，而不是使用默认名称Java_java_lang_Thread_getClass

二. Thread实现Runnable接口。如果自定义一个线程class，有两种方法：

1.继承Thread，并重载run方法。

 * <p><hr><blockquote><pre>
 *     class PrimeThread extends Thread {
 *         long minPrime;
 *         PrimeThread(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 * 
 *         public void run() {
 *             // compute primes larger than minPrime
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote><hr>
 * <p>
 * The following code would then create a thread and start it running: 
 * <p><blockquote><pre>
 *     PrimeThread p = new PrimeThread(143);
 *     p.start();
 * </pre></blockquote>
 * <p>

2.实现Runnable接口。也需要实现run方法

 * <p><hr><blockquote><pre>
 *     class PrimeRun implements Runnable {
 *         long minPrime;
 *         PrimeRun(long minPrime) {
 *             this.minPrime = minPrime;
 *         }
 * 
 *         public void run() {
 *             // compute primes larger than minPrime
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote><hr>
 * <p>
 * The following code would then create a thread and start it running: 
 * <p><blockquote><pre>
 *     PrimeRun p = new PrimeRun(143);
 *     new Thread(p).start();
 * </pre></blockquote>
 * <p>

第一种方法，PrimeThread在调用start方法的时候，会把当前的线程加入到线程组中，JVM在native中执行run。

    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 || this != me)
            throw new IllegalThreadStateException();
        group.add(this);
        start0();
        if (stopBeforeStart) {
        stop0(throwableFromStop);
    }
    }

第二种方法，PrimeRun新建一个对象后，需要把这个对象引用到Thread对象中，再调用start方法。

    public Thread(Runnable target) {
    init(null, target, "Thread-" + nextThreadNum(), 0);
    }

    public void run() {
    if (target != null) {
        target.run();
    }
    }

三. 在Thread初始化过程中

    /**
     * Initializes a Thread.
     *
     * @param g the Thread group
     * @param target the object whose run() method gets called
     * @param name the name of the new Thread
     * @param stackSize the desired stack size for the new thread, or
     *        zero to indicate that this parameter is to be ignored.
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
    Thread parent = currentThread();
    SecurityManager security = System.getSecurityManager();
    if (g == null) {
        /* Determine if it's an applet or not */
        
        /* If there is a security manager, ask the security manager
           what to do. */
        if (security != null) {
        g = security.getThreadGroup();
        }

        /* If the security doesn't have a strong opinion of the matter
           use the parent thread group. */
        if (g == null) {
        g = parent.getThreadGroup();
        }
    }

    /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
    g.checkAccess();

    /*
     * Do we have the required permissions?
     */
    if (security != null) {
        if (isCCLOverridden(getClass())) {
            security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
        }
    }


        g.addUnstarted();

    this.group = g;
    this.daemon = parent.isDaemon();
    this.priority = parent.getPriority();
    this.name = name.toCharArray();
    if (security == null || isCCLOverridden(parent.getClass()))
        this.contextClassLoader = parent.getContextClassLoader();
    else
        this.contextClassLoader = parent.contextClassLoader;
    this.inheritedAccessControlContext = AccessController.getContext();
    this.target = target;
    setPriority(priority);
        if (parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
        ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();

        this.me = this;
    }

这个方法大意就是通过安全管理器确认当前线程的权限，另外通过这个初始化，可以看出当前线程继承了父线程的DaemonThread(或者Non-DaemonThread)和Priority。