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。
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。
