java Thread源码分析

1、Runnable接口源码：

public interface Runnable {
public abstract void run();
}

2、Thread类与Runnable接口的继承关系

public class Thread implements Runnable{

}

3、构造函数

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

还有其它的构造方法，此处省略。。。

　　这里的第三个参数是设置线程的名称，从下面的代码中可以看出，生成名称的规则是：”Thread-”加上创建的线程的个数（第几个）。

继续查看init方法：

/**
* 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;
//每个线程都有一个优先级，高优先级线程的执行优先于低优先级线程。每个线程都可以或不可以标记为一个守护程序。当某个线程中运行的代码创建一个新对象时，该新线程的初始优先级被设定为创建线程的优先级，并且当且仅当创建线程是守护线程时，新线程才是守护程序。
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();
}

初始化时设置了是否为守护线程，优先级，初始化名称。

4、Thread的start方法的实现：

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();
group.add(this);
start0();
if (stopBeforeStart) {
stop0(throwableFromStop);
}
}

这里主要的是start0方法；查看其实现：

private native void start0();

这里使用了本地调用，通过C代码初始化线程需要的系统资源。可见，线程底层的实现是通过C代码去完成的。

Thread的run方法的实现

/**
* If this thread was constructed using a separate
* <code>Runnable</code> run object, then that
* <code>Runnable</code> object's <code>run</code> method is called;
* otherwise, this method does nothing and returns.
* <p>
* Subclasses of <code>Thread</code> should override this method.
*
* @see     #start()
* @see     #stop()
* @see     #Thread(ThreadGroup, Runnable, String)
*/
public void run() {
if (target != null) {
target.run();
}
}

这里的target实际上要保存的是一个Runnable接口的实现的引用：

private Runnable target;

        所以使用继承Thread创建线程类时，需要重写run方法，因为默认的run方法什么也不干。

　　而当我们使用Runnable接口实现线程类时，为了启动线程，需要先把该线程类实例初始化一个Thread，实际上就执行了如下构造函数：

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

　　即是把线程类的引用保存到target中。这样，当调用Thread的run方法时，target就不为空了，而是继续调用了target的run方法，所以我们需要实现Runnable的run方法。这样通过Thread的run方法就调用到了Runnable实现类中的run方法。

　　这也是Runnable接口实现的线程类需要这样启动的原因。

5、sleep(long millis, int nanos) 方法，原来nanos不管用，只能精确到毫秒。

/**
* Causes the currently executing thread to sleep (cease execution)
* for the specified number of milliseconds plus the specified number
* of nanoseconds, subject to the precision and accuracy of system
* timers and schedulers. The thread does not lose ownership of any
* monitors.
*
* @param      millis   the length of time to sleep in milliseconds.
* @param      nanos    0-999999 additional nanoseconds to sleep.
* @exception  IllegalArgumentException  if the value of millis is
*             negative or the value of nanos is not in the range
*             0-999999.
* @exception  InterruptedException if any thread has interrupted
*             the current thread.  The <i>interrupted status</i> of the
*             current thread is cleared when this exception is thrown.
* @see        Object#notify()
*/
public static void sleep(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}

if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}

if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}

sleep(millis);
}

转载自：http://www.xuebuyuan.com/460311.html

参考：

http://czj4451.iteye.com/blog/1770003

http://wdhdmx.iteye.com/blog/1202379

http://blog.csdn.net/arkblue/article/details/7742795