1、Runnable接口源码:
1 public interface Runnable {
2 public abstract void run();
3 }
2、Thread类与Runnable接口的继承关系 1 public class Thread implements Runnable{
2
3 }
Runnable接口仅有一个run()方法,Thread类实现了Runnable接口,所以,Thread类也实现了Runnable接口。 3、构造函数 1 public Thread() {
2 init(null, null, "Thread-" + nextThreadNum(), 0);
3 }
1 public Thread(Runnable target) {
2 init(null, target, "Thread-" + nextThreadNum(), 0);
3 }
1 public Thread(ThreadGroup group, Runnable target) {
2 init(group, target, "Thread-" + nextThreadNum(), 0);
3 }
1 public Thread(String name) {
2 init(null, null, name, 0);
3 }
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: 1 /**
2 * Initializes a Thread.
3 *
4 * @param g the Thread group
5 * @param target the object whose run() method gets called
6 * @param name the name of the new Thread
7 * @param stackSize the desired stack size for the new thread, or
8 * zero to indicate that this parameter is to be ignored.
9 */
//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 47 this.daemon = parent.isDaemon();
48 this.priority = parent.getPriority();
49 this.name = name.toCharArray();
50 if (security == null || isCCLOverridden(parent.getClass()))
51 this.contextClassLoader = parent.getContextClassLoader();
52 else
53 this.contextClassLoader = parent.contextClassLoader;
54 this.inheritedAccessControlContext = AccessController.getContext();
55 this.target = target;
56 setPriority(priority);
57 if (parent.inheritableThreadLocals != null)
58 this.inheritableThreadLocals =
59 ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
60 /* Stash the specified stack size in case the VM cares */
61 this.stackSize = stackSize;
62
63 /* Set thread ID */
64 tid = nextThreadID();
65 }
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: 1 public synchronized void start() {
2 /**
3 * This method is not invoked for the main method thread or "system"
4 * group threads created/set up by the VM. Any new functionality added
5 * to this method in the future may have to also be added to the VM.
6 *
7 * A zero status value corresponds to state "NEW".
8 */
9 if (threadStatus != 0)
10 throw new IllegalThreadStateException();
11 group.add(this);
12 start0();
13 if (stopBeforeStart) {
14 stop0(throwableFromStop);
15 }
16 }
这里主要的是start0方法;查看其实现: 1 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。 4、Thread的run方法的实现 1 public void run() {
2 if (target != null) {
3 target.run();
4 }
5 }
这里的target实际上要保存的是一个Runnable接口的实现的引用: 1 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: 1 public Thread(Runnable target) {
2 init(null, target, "Thread-" + nextThreadNum(), 0);
3 }
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。 |
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