1、Runnable接口源碼：
1 public interface Runnable { 2 public abstract void run(); 3 }
2、Thread類與Runnable接口的繼承關系
1 public class Thread implements Runnable{ 2
3 }
Runnable接口僅有一個run()方法，Thread類實現了Runnable接口，所以，Thread類也實現了Runnable接口。
3、構造函數
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 }
還有其它的構造方法，此處省略。。。
這里的第三個參數是設置線程的名稱，從下面的代碼中可以看出，生成名稱的規則是：”Thread-”加上創建的線程的個數（第幾個）。
繼續查看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：線程組表示一個線程的集合。此外，線程組也可以包含其他線程組。線程組構成一棵樹，在樹中，除了初始線程組外，每個線程組都有一個父線程組。
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;
//每個線程都有一個優先級，高優先級線程的執行優先於低優先級線程。每個線程都可以或不可以標記為一個守護程序。當某個線程中運行的代碼創建一個新 Thread 對象時，該新線程的初始優先級被設定為創建線程的優先級，並且當且僅當創建線程是守護線程時，新線程才是守護程序。
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 }
初始化時設置了是否為守護線程，優先級，初始化名稱。
4、Thread的start方法的實現：
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 }
這里主要的是start0方法；查看其實現：
1 private native void start0();
這里使用了本地調用，通過C代碼初始化線程需要的系統資源。可見，線程底層的實現是通過C代碼去完成的。
4、Thread的run方法的實現
1 public void run() { 2 if (target != null) { 3 target.run(); 4 } 5 }
這里的target實際上要保存的是一個Runnable接口的實現的引用：
1 private Runnable target;
所以使用繼承Thread創建線程類時，需要重寫run方法，因為默認的run方法什么也不干。
而當我們使用Runnable接口實現線程類時，為了啟動線程，需要先把該線程類實例初始化一個Thread，實際上就執行了如下構造函數：
1 public Thread(Runnable target) { 2 init(null, target, "Thread-" + nextThreadNum(), 0); 3 }
即是把線程類的引用保存到target中。這樣，當調用Thread的run方法時，target就不為空了，而是繼續調用了target的run方法，所以我們需要實現Runnable的run方法。這樣通過Thread的run方法就調用到了Runnable實現類中的run方法。
這也是Runnable接口實現的線程類需要這樣啟動的原因。