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  通过ThreadLocal和InheritableThreadLocal，我们能够很方便的设计出线程安全的类。JDK底层是如何做到的呢？ThreadLocal和InheritableThreadLocal有什么区别呢与联系呢？为什么有了ThreadLocal类还需要InheritableThreadLocal类，他们与Thread类是什么关系？带着这些问题我们来分析他们的源码。

ThreadLocal

  ThreadLocal是为每一个线程创建一个单独的变量副本，每个线程都可以独立地改变自己所拥有的变量副本，而不会影响其他线程所对应的副本。ThreadLocal为多线程环境下变量问题提供了另外一种解决思路。下面来看看ThreadLocal内部是如何做到的。下面分别分析ThreadLocal提供的方法。

• set方法源码解析

public class ThreadLocal<T> {/**
**传入一个value参数
**/public void set(T value) {//首先得到当前线程对象Thread t = Thread.currentThread();//根据当前线程，得到ThreadLocalMap 引用。ThreadLocalMap map = getMap(t);//如果map不为空，则把值set到map里if (map != null)map.set(this, value);else
//map为空，则创建一个map对象，并设置值createMap(t, value);}//根据Thread得到 ThreadLocalMap 对象ThreadLocalMap getMap(Thread t) {return t.threadLocals;}//map为空的话，会新建一个ThreadLocalMap,并将引用交给当前Thread的threadLocals 变量void createMap(Thread t, T firstValue) {t.threadLocals = new ThreadLocalMap(this, firstValue);}}

ThreadLocal对象关系图

• get方法源码解析

public class ThreadLocal<T> {public T get() {//得到当前线程Thread t = Thread.currentThread();//这里取的是Thread里的ThreadLocalMap引用ThreadLocalMap map = getMap(t);if (map != null) {
//从map里取值，key为ThreadLocal对象ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;}
//map为空的话，会调用setInitialValue方法return setInitialValue();}//private T setInitialValue() {
//得到一个默认的值，initialValue为protected，留给子类实现，T value = initialValue();
//得到当前线程Thread t = Thread.currentThread();
//再次尝试得到map对象ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);else
//调用createMap方法createMap(t, value);return value;}protected T initialValue() {return null;}}

get方法无非就是得到当前线程的ThreadLocalMap，如果不为空，则根据ThreadLocal为key进行取值，如果为空，则会调用createMap为Thread线程构造一个ThreadLocalMap对象，并返回initialValue方法的值。

InheritableThreadLocal

  InheritableThreadLocal用于子线程能够拿到父线程往ThreadLocal里设置的值。使用代码如下：

public class Test {public static ThreadLocal<Integer> threadLocal = new InheritableThreadLocal<Integer>();public static void main(String args[]) {threadLocal.set(new Integer(456));Thread thread = new MyThread();thread.start();System.out.println("main = " + threadLocal.get());}static class MyThread extends Thread {@Overridepublic void run() {System.out.println("MyThread = " + threadLocal.get());}}
}

输出结果

如果把上面的InheritableThreadLocal换成ThreadLocal的话，在子线程里的输出将为是空。从上面的代码里也可以看出InheritableThreadLocal是ThreadLocal的子类。下面是InheritableThreadLocal的源码：

package java.lang;
import java.lang.ref.*;public class InheritableThreadLocal<T> extends ThreadLocal<T> {//这个方法留给子类实现protected T childValue(T parentValue) {return parentValue;}//重写getMap方法，返回的是Thread的inheritableThreadLocals引用ThreadLocalMap getMap(Thread t) {return t.inheritableThreadLocals;}//重写createMap方法，构造的ThreadLocalMap会传给Thread的inheritableThreadLocals 变量void createMap(Thread t, T firstValue) {t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);}
}

从源码上看，跟ThreadLocal不一样的无非是ThreadLocalMap的引用不一样了，从逻辑上来讲，这并不能做到子线程得到父线程里的值。那秘密在那里呢？通过跟踪Thread的构造方法，你能够发现是在构造Thread对象的时候对父线程的InheritableThreadLocal进行了复制。下面是Thread的部分源码：

public class Thread implements Runnable {//默认人构造方法，会调用init方法进行初使化public Thread() {init(null, null, "Thread-" + nextThreadNum(), 0);}private void init(ThreadGroup g, Runnable target, String name,long stackSize) {init(g, target, name, stackSize, null);}//最终会调用到当前这个方法private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc) {if (name == null) {throw new NullPointerException("name cannot be null");}this.name = name.toCharArray();
// parent为当前线程，也就是调用了new Thread();方法的线程Thread parent = currentThread();SecurityManager security = System.getSecurityManager();if (g == null) {if (security != null) {g = security.getThreadGroup();}if (g == null) {g = parent.getThreadGroup();}}g.checkAccess();if (security != null) {if (isCCLOverridden(getClass())) {security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);}}g.addUnstarted();this.group = g;
//在这里会继承父线程是否为后台线程的属性还有父线程的优先级this.daemon = parent.isDaemon();this.priority = parent.getPriority();if (security == null || isCCLOverridden(parent.getClass()))this.contextClassLoader = parent.getContextClassLoader();elsethis.contextClassLoader = parent.contextClassLoader;this.inheritedAccessControlContext =acc != null ? acc : AccessController.getContext();this.target = target;setPriority(priority);
//这里是重点，当父线程的inheritableThreadLocals 不为空的时候，会调用 ThreadLocal.createInheritedMap方法，传入的是父线程的inheritableThreadLocals。原来复制变量的秘密在这里if (parent.inheritableThreadLocals != null)this.inheritableThreadLocals =ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);/* Stash the specified stack size in case the VM cares */this.stackSize = stackSize;/* Set thread ID */tid = nextThreadID();}}

通过跟踪Thread的构造方法，我们发现只要父线程在构造子线程（调用new Thread()）的时候inheritableThreadLocals变量不为空。新生成的子线程会通过ThreadLocal.createInheritedMap方法将父线程inheritableThreadLocals变量有的对象复制到子线程的inheritableThreadLocals变量上。这样就完成了线程间变量的继承与传递。
ThreadLocal.createInheritedMap方法的源码如下：

public class ThreadLocal<T> {
//根据传入的map，构造一个新的ThreadLocalMapstatic ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap);}static class ThreadLocalMap {
//这个private的构造方法就是专门给ThreadLocal使用的private ThreadLocalMap(ThreadLocalMap parentMap) {
//ThreadLocalMap还是用Entry数组来存储对象的Entry[] parentTable = parentMap.table;int len = parentTable.length;setThreshold(len);table = new Entry[len];
//这里是复制parentMap数据的逻辑for (int j = 0; j < len; j++) {Entry e = parentTable[j];if (e != null) {ThreadLocal key = e.get();if (key != null) {Object value = key.childValue(e.value);Entry c = new Entry(key, value);int h = key.threadLocalHashCode & (len - 1);while (table[h] != null)h = nextIndex(h, len);table[h] = c;size++;}}}}}}

总结：

• ThreadLocal和InheritableThreadLocal本质上只是为了方便编码给的工具类，具体存数据是ThreadLocalMap 对象。
• ThreadLocalMap 存的key对象是ThreadLocal，value就是真正需要存的业务对象。
• Thread里通过两个变量持用ThreadLocalMap 对象，分别为：threadLocals和inheritableThreadLocals。
• InheritableThreadLocal之所以能够完成线程间变量的传递，是在new Thread()的时候对inheritableThreadLocals对像里的值进行了复制。
• 子线程通过继承得到的InheritableThreadLocal里的值与父线程里的InheritableThreadLocal的值具有相同的引用，如果父子线程想实现不影响各自的对象，可以重写InheritableThreadLocal的childValue方法。