本来想看 ThreadLocal 的源码的，但发现其中最重要的 get/set 方法都是操纵的 Thread 类 中的 threadLocals 变量 (java.lang.ThreadLocal.ThreadLocalMap)，索性先来看一下 Thread 的源码吧，可以留意一下其中与 ThreadLocal 相关的属性，这样下次阅读 ThreadLocal 的核心 API 时，就能够轻易理解其原理咯。不多 BB，直接上硬菜。 实现多线程从本质上都是由 Thread 类 来完成的，其源码量很多，本次只看一些常见且重要的部分，源码和解析如下。 ```java public class Thread implements Runnable { /** 这里只看一些 常见的参数 */ /** 线程名 */ private volatile char name[]; /** 优先级 */ private int priority; /** 是否为守护线程 */ private boolean daemon; /** 线程要执行的目标任务 */ private Runnable target; /** 所属线程组 */ private ThreadGroup group; /** 类加载器 */ private ClassLoader contextClassLoader; /** * ThreadLocal 能为线程设置线程私有变量 就是通过下面这个threadLocals变量完成的， * ThreadLocal的get/set方法就是通过操作 各个线程的 threadLocals 变量实现的。 * 1、线程A持有一个 ThreadLocalMap 变量； * 2、线程A调用一个类的 ThreadLocal变量 tlA 的 get/set方法； * 3、tlA（ThreadLocal）的 get/set方法 获取当前线程A，调用 线程A 的 ThreadLocalMap变量 的get/put方法； * 4、其它线程 调用 tlA（ThreadLocal）的 get/set方法 同理。 */ ThreadLocal.ThreadLocalMap threadLocals; ThreadLocal.ThreadLocalMap inheritableThreadLocals; /** 线程栈的大小 */ private long stackSize; /** * Thread类定义了6个线程状态：New、Runnable、Blocked、Waiting、TimedWaiting、Terminated(终止) * 实际上还会把 Runnable 再细分为 就绪(未抢到时间片) 和 运行中(抢到时间片) */ private volatile int threadStatus; /** 最小优先级 */ public static final int MIN_PRIORITY = 1; /** 中等优先级 */ public static final int NORM_PRIORITY = 5; /** 最大优先级 */ public static final int MAX_PRIORITY = 10; /** * 内部枚举类，用来描述线程状态，状态值有： * NEW： 新建，还未调用start()方法； * RUNNABLE： 运行，在java多线程模型中，就绪和运行都是运行状态； * BLOCKED： 阻塞； * WAITING： 等待，需要其他的线程来唤醒； * TIMED_WAITING：超时等待，可以在指定的时间内自动醒来，如 sleep()方法； * TERMINATED： 终止，线程执行完毕。 */ public static final class State extends Enum { public static final State NEW; public static final State RUNNABLE; public static final State BLOCKED; public static final State WAITING; public static final State TIMED_WAITING; public static final State TERMINATED; private static final State VALUES[]; static { NEW = new State("NEW", 0); RUNNABLE = new State("RUNNABLE", 1); BLOCKED = new State("BLOCKED", 2); WAITING = new State("WAITING", 3); TIMED_WAITING = new State("TIMED_WAITING", 4); TERMINATED = new State("TERMINATED", 5); VALUES = (new State[] { NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED }); } private State(String s, int i) { super(s, i); } } /** * 一系列 构造方法 ------------------------------------------------------ * 可以看出来，其中都调用了init()方法，这也是一个约定俗成的规矩， 即，如果要在 new 时进行一些初始化操作， * 那么请将初始化操作单独写在 init()方法中，然后在构造函数中调用该 init()方法 */ public Thread() { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(null, null, (new StringBuilder()).append("Thread-").append(nextThreadNum()).toString(), 0L); } public Thread(Runnable runnable) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(null, runnable, (new StringBuilder()).append("Thread-").append(nextThreadNum()).toString(), 0L); } Thread(Runnable runnable, AccessControlContext accesscontrolcontext) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(null, runnable, (new StringBuilder()).append("Thread-").append(nextThreadNum()).toString(), 0L, accesscontrolcontext); } public Thread(ThreadGroup threadgroup, Runnable runnable) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(threadgroup, runnable, (new StringBuilder()).append("Thread-").append(nextThreadNum()).toString(), 0L); } public Thread(String s) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(null, null, s, 0L); } public Thread(ThreadGroup threadgroup, String s) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(threadgroup, null, s, 0L); } public Thread(Runnable runnable, String s) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(null, runnable, s, 0L); } public Thread(ThreadGroup threadgroup, Runnable runnable, String s) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(threadgroup, runnable, s, 0L); } public Thread(ThreadGroup threadgroup, Runnable runnable, String s, long l) { daemon = false; stillborn = false; threadLocals = null; inheritableThreadLocals = null; threadStatus = 0; blockerLock = new Object(); init(threadgroup, runnable, s, l); } private void init(ThreadGroup threadgroup, Runnable runnable, String s, long l) { init(threadgroup, runnable, s, l, null); } /** * 初始化线程 */ private void init(ThreadGroup threadgroup, Runnable runnable, String name, long l, AccessControlContext accesscontrolcontext) { // 参数校验，线程name不能为null if (name == null) throw new NullPointerException("name cannot be null"); this.name = name.toCharArray(); // 当前线程就是该线程的父线程 Thread parent = currentThread(); SecurityManager securitymanager = System.getSecurityManager(); if (threadgroup == null) { if (securitymanager != null) threadgroup = securitymanager.getThreadGroup(); if (threadgroup == null) threadgroup = parent.getThreadGroup(); } threadgroup.checkAccess(); if (securitymanager != null && isCCLOverridden(getClass())) securitymanager.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION); threadgroup.addUnstarted(); // 守护线程、优先级等设置为父线程的对应属性 group = threadgroup; daemon = parent.isDaemon(); priority = parent.getPriority(); if (securitymanager == null || isCCLOverridden(parent.getClass())) contextClassLoader = parent.getContextClassLoader(); else contextClassLoader = parent.contextClassLoader; inheritedAccessControlContext = accesscontrolcontext == null ? AccessController.getContext() : accesscontrolcontext; target = runnable; setPriority(priority); if (parent.inheritableThreadLocals != null) // 创建线程共享变量副本 inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); stackSize = l; // 分配线程id tid = nextThreadID(); } public synchronized void start() { //假若当前线程初始化还未做好，不能start，0->NEW状态 if (threadStatus != 0) throw new IllegalThreadStateException(); //通知group该线程即将启动，group的未启动线程数量减1 group.add(this); boolean started = false; try { // 调用native的start0()方法 启动线程，启动后执行run()方法 start0(); started = true; } finally { try { //启动不成功，group设置当前线程启动失败 if (!started) { group.threadStartFailed(this); } } catch (Throwable ignore) { } } } private native void start0(); public void run() { if (target != null) target.run(); } /** * 请求终止线程。interrupt不会真正停止一个线程，它仅仅是给这个线程发了一个信号， * 告诉它要结束了，具体要中断还是继续运行，将由被通知的线程自己处理 */ public void interrupt() { if (this != Thread.currentThread()) checkAccess(); synchronized (blockerLock) { Interruptible b = blocker; if (b != null) { interrupt0(); b.interrupt(this); return; } } interrupt0(); } private native void interrupt0(); /** * 线程main 调用了线程A的join方法，则 线程main 会被阻塞，直到线程A执行完毕 */ public final void join() throws InterruptedException { join(0); } /** * 实际上是利用 wait/notify机制 来实现的 */ public final synchronized void join(long millis) throws InterruptedException { long base = System.currentTimeMillis(); long now = 0; if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } // millis 为 0，所以走这个分支 if (millis == 0) { // 当前线程是否还在运行，还在运行 则main线程 进入等待状态，直到 A线程运行完毕，将其唤醒 while (isAlive()) { wait(0); } } else { while (isAlive()) { long delay = millis - now; if (delay <= 0) { break; } wait(delay); now = System.currentTimeMillis() - base; } } } /** * 线程睡眠指定的时间，释放CPU资源，但不释放锁 */ public static native void sleep(long millis) throws InterruptedException; /** * 线程是否还在运行 */ public final native boolean isAlive(); } ``` 之前一直对线程状态 及 状态切换的概念模糊不清，现在通过源码中对线程状态的定义，我们可以画张图来重新回顾一下，以使我们对其有更加深刻的理解。 ![avatar](../../../images/JDK1.8/ThreadStatusChange.png)