InheritableThreadLocal：子线程继承父线程的本地变量

朴灿烈づ我的快乐病毒、 2023-02-21 12:46 16阅读 0赞

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![在这里插入图片描述][20200515113638671.jpg_pic_center]

# 线程本地变量相关的博客目录 #

*  [ThreadLocal内存泄露原因分析][ThreadLocal]
 *  [ThreadLocal 内存泄漏 代码演示 实例演示][ThreadLocal _ _]
 *  [InheritableThreadLocal：子线程继承父线程的本地变量][InheritableThreadLocal]
 *  [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][InheritableThreadLocal 1]
 *  [transmittable-thread-local:解决线程池之间ThreadLocal本地变量传递的问题][transmittable-thread-local_ThreadLocal]

# 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。

[20200515113638671.jpg_pic_center]: https://img-blog.csdnimg.cn/20200515113638671.jpg#pic_center
[ThreadLocal]: https://blog.csdn.net/liubenlong007/article/details/107046897
[ThreadLocal _ _]: https://blog.csdn.net/liubenlong007/article/details/107048843
[InheritableThreadLocal]: https://blog.csdn.net/liubenlong007/article/details/107049375
[InheritableThreadLocal 1]: https://blog.csdn.net/liubenlong007/article/details/107049975
[transmittable-thread-local_ThreadLocal]: https://editor.csdn.net/md/?articleId=107050929