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 }
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：
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 */
//ThreadGroup：线程组表示一个线程的集合。此外，线程组也可以包含其他线程组。线程组构成一棵树，在树中，除了初始线程组外，每个线程组都有一个父线程组。
10 private void init(ThreadGroup g, Runnable target, String name,
11 long stackSize) {
12 Thread parent = currentThread();
13 SecurityManager security = System.getSecurityManager();
14 if (g == null) {
15 /* Determine if it's an applet or not */
16
17 /* If there is a security manager, ask the security manager
18 what to do. */
19 if (security != null) {
20 g = security.getThreadGroup();
21 }
22
23 /* If the security doesn't have a strong opinion of the matter
24 use the parent thread group. */
25 if (g == null) {
26 g = parent.getThreadGroup();
27 }
28 }
29
30 /* checkAccess regardless of whether or not threadgroup is
31 explicitly passed in. */
32 g.checkAccess();
33
34 /*
35 * Do we have the required permissions?
36 */
37 if (security != null) {
38 if (isCCLOverridden(getClass())) {
39 security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
40 }
41 }
42
43
44 g.addUnstarted();
45
46 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 Thread 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。
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。