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线程本地变量相关的博客目录

ThreadLocal内存泄露原因分析ThreadLocal 内存泄漏 代码演示 实例演示InheritableThreadLocal：子线程继承父线程的本地变量当InheritableThreadLocal遇到线程池：主线程本地变量修改后，子线程无法读取到新值transmittable-thread-local:解决线程池之间ThreadLocal本地变量传递的问题

概述

ThreadLocal可以保存一些变量仅供当前线程使用，其他线程不可见。 实际工作中可能会由于任务复杂，父线程创建几个子线程并发致性任务，那么父线程的本地变量如何传递到子线程呢? 答案是使用InheritableThreadLocal。

代码实例

public static ThreadLocal<Integer> threadLocal = new InheritableThreadLocal<>(); public static void main(String[] args) throws Exception { threadLocal.set(12345); Thread thread = new MyThread(); thread.start(); System.out.println("main = " + threadLocal.get()); } static class MyThread extends Thread { @Override public void run() { System.out.println("MyThread = " + threadLocal.get()); } }

运行结果：

main = 12345 MyThread = 12345

子线程中获取到了父线程的变量值。

如果将InheritableThreadLocal换为ThreadLocal，则子线程打印结果为MyThread = null，读者自行验证。

源码分析

public class InheritableThreadLocal<T> extends ThreadLocal<T> { /** * 重写了childValue方法 * 父线程创建子线程时，向子线程复制InheritableThreadLocal变量时使用 */ protected T childValue(T parentValue) { return parentValue; } //注意这里使用的是Thread对象里的inheritableThreadLocals变量，而不是threadLocals ThreadLocalMap getMap(Thread t) { return t.inheritableThreadLocals; } //将父线程的threadLocal变量拷贝到子线程 void createMap(Thread t, T firstValue) { t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue); } }

当我们在new Thread()时，会有从父线程拷贝线程本地变量到子线程的代码：

public Thread() { this(null, null, "Thread-" + nextThreadNum(), 0); } public Thread(ThreadGroup group, Runnable target, String name, long stackSize) { this(group, target, name, stackSize, null, true); } private Thread(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) { //省略... if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); //省略 }

默认情况下，inheritThreadLocals值为true，parent.inheritableThreadLocals也不为空，所以会执行ThreadLocal.createInheritedMap方法。

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 (Entry e : parentTable) { 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++; } } } }

这段代码比较简单，就是在创建子线程时逐个读取父线程的本地变量，赋值给子线程本地变量。