如何解析InheritableThreadLocal

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InheritableThreadLocal 继承自ThreadLocal，重写了childValue、getMap、createMap 方法，主要作用是子线程能够读取父线程的变量 看下这个类

public class InheritableThreadLocal extends ThreadLocal {    protected T childValue(T parentValue) {        return parentValue;    }    //返回的是Thread类的inheritableThreadLocals，而ThreadLocal使用的是threadLocals变量    ThreadLocalMap getMap(Thread t) {       return t.inheritableThreadLocals;    }    void createMap(Thread t, T firstValue) {        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);    }}

示例：

public class InheritableThreadLocalTest {    private static final InheritableThreadLocal threadLocal = new InheritableThreadLocal ();    public static void main(String [] args) throws InterruptedException {        threadLocal.set("hello world");        Thread thread = new Thread(new Runnable() {            @Override            public void run() {                //threadLocal.set("son thread");                System.out.println("thread:" + threadLocal.get());            }        });        thread.start();        thread.join();        System.out.println("main:" + threadLocal.get());    }}

输出：

thread:hello worldmain:hello world

这里如果我在子线程中set了一个新值，那结果会怎么样？ 发现父线程的值没有改变

thread:son threadmain:hello world

源码剖析

1. 首先从新建子线程开始分析，这里主要就是将父线程的值copy到子线程中

//构造函数public Thread(Runnable target) {    init(null, target, "Thread-" + nextThreadNum(), 0);}//直接跳到，最终的init方法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();    //....省略中间部分，看主要的    //获取父线程的inheritableThreadLocals变量，如果不为空就copy父线程中的变量到子线程    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();}//ThreadLocal.createInheritedMap方法static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {    return new ThreadLocalMap(parentMap);}    //ThreadLocalMap(parentMap) 方法private ThreadLocalMap(ThreadLocalMap parentMap) {    Entry[] parentTable = parentMap.table;    int len = parentTable.length;    setThreshold(len);    //新建一个Entry数组，Entry继承了WeakReference，key为ThreadLocal类型    //这是为了在大数据量的时候，方便GC来回收已经失效的数据    table = new Entry[len];    for (int j = 0; j < len; j++) {        Entry e = parentTable[j];        if (e != null) {            @SuppressWarnings("unchecked")            ThreadLocal