ThreadLocal是一个线程局部变量类，他是跟当前线程绑定的，线程与线程之间是相互不影响的。也就是在一个线程中通过ThreadLocal进行set值之后，其他线程get是get不到的，只能自己线程进行get。 但是有时候我们想主线程set值，子线程都能取到，这样的话，ThreadLocal就不适用了。幸好java提供了一个类InheritableThreadLocal，它继承自ThreadLocal， 而且Thread类中也定义了相关的变量，使得这个场景得以实现。
直接上测试代码了：
static ThreadLocal<String> threadLocal = new ThreadLocal<>();static InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();public static void main(String[] args) {threadLocal.set("abc");new Thread(new Runnable() {@Overridepublic void run() {System.out.println(Thread.currentThread().getName()+"====="+threadLocal.get());}},"thread1").start();inheritableThreadLocal.set("zhangsan");new Thread(new Runnable() {@Overridepublic void run() {System.out.println(Thread.currentThread().getName()+"====="+inheritableThreadLocal.get());}},"thread2").start();}通过以上代码可以看出，thread1输出的是null， thread2输出的是zhangsan。
其实现原理也很简单。
InheritableThreadLocal重写了ThreadLocal的三个方法：
childValue，getMap，createMap。
源码如下：
public class InheritableThreadLocal<T> extends ThreadLocal<T> {protected T childValue(T parentValue) {return parentValue;}ThreadLocalMap getMap(Thread t) {return t.inheritableThreadLocals;}void createMap(Thread t, T firstValue) {t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);}}
当主线程调用 inheritableThreadLocal.set("zhangsan");
之后，调用的实际上是ThreadLocal的set方法，源码如下：
public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}可以看到set方法会先getMap，get不到就会createMap。主线程第一次调用getMap的时候，主线程对象里面的
inheritableThreadLocals引用默认指向的是null，所以第一次set的时候会执行createMap，因为实例化对象是inheritableThreadLocal对象，所以实际调用的也是他自己的createMap方法。
其实主线程里的set就做了一个事，就是对当前线程里面的inheritableThreadLocal变量进行实例化。
然后主线程里new Thread进行创建子线程的时候，这个时候会执行Thread的init方法，源码如下：
public Thread(Runnable target, String name) {init(null, target, name, 0);
}private void init(ThreadGroup g, Runnable target, String name,long stackSize) {init(g, target, name, stackSize, null, true);
}private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc,boolean inheritThreadLocals) {if (name == null) {throw new NullPointerException("name cannot be null");}this.name = name;Thread parent = currentThread();SecurityManager security = System.getSecurityManager();if (g == null) {/* Determine if it's an applet or not *//* If there is a security manager, ask the security managerwhat to do. */if (security != null) {g = security.getThreadGroup();}/* If the security doesn't have a strong opinion of the matteruse the parent thread group. */if (g == null) {g = parent.getThreadGroup();}}/* checkAccess regardless of whether or not threadgroup isexplicitly passed in. */g.checkAccess();/** Do we have the required permissions?*/if (security != null) {if (isCCLOverridden(getClass())) {security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);}}g.addUnstarted();this.group = g;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);if (inheritThreadLocals && 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();}在init方法里面有一段代码
这里的parent是针对要创建的子线程而言的，实际是 Thread parent = currentThread(); ，在这里就是主线程本身。
调用了ThreadLocal的createInheritedMap方法，源码如下：
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap);}private ThreadLocalMap(ThreadLocalMap parentMap) {Entry[] parentTable = parentMap.table;int len = parentTable.length;setThreshold(len);table = new Entry[len];for (int j = 0; j < len; j++) {Entry e = parentTable[j];if (e != null) {@SuppressWarnings("unchecked")ThreadLocal<Object> key = (ThreadLocal<Object>) 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++;}}}}ThreadLocalMap是ThreadLocal的内部静态类。在ThreadLocalMap的构造函数中可以看出，这个是private的，其内部有一段代码是 Object value = key.childValue(e.value); 这个key就是ThreadLocal对象，在本示例中，就是inheritableThreadLocal对象，
也就是说调用了调用了inheritableThreadLocal的childValue方法进行获取到值，然后创建entry对象，放到map树里。
至此inheritableThreadLocal重写的三个方法都已经被调用了。
我们可以通过重写InheritableThreadLocal的childValue来改变get方法获取的值。
示例代码如下：
public class TestThreadLocal {static InheritableThreadLocal<String> inheritableThreadLocal1 = new MyThreadLocal<> ("wocaolai");public static void main(String[] args) {inheritableThreadLocal1.set("zhangsan");new Thread(new Runnable() {@Overridepublic void run() {System.out.println(Thread.currentThread().getName()+"====="+inheritableThreadLocal1.get());}},"thread3").start();}static class MyThreadLocal<T> extends InheritableThreadLocal<T> {public T myvalue;protected T childValue(T parentValue) {return myvalue;}public MyThreadLocal(T myvalue) {this.myvalue = myvalue;}}}输出的结果是
而不是张三了。