Java InheritableThreadLocal用法详细介绍
作者:IT利刃出鞘
简介
本文介绍InheritableThreadLocal的用法。
ThreadLocal可以将数据绑定当前线程,如果希望当前线程的ThreadLocal的数据被子线程使用,实现方式就会相当困难(需要用户自己在代码中传递)。
InheritableThreadLocal可以方便地让子线程自动获取父线程ThreadLocal的数据。
ThreadLocal和InheritableThreadLocal都要注意,用完后要调用其remove()方法,不然可能导致内存泄露或者产生脏数据。
问题复现
代码
package com.example.a; public class Demo { private static ThreadLocal<String> threadLocal = new ThreadLocal<>(); public static void main(String[] args) { threadLocal.set("hello"); System.out.println("主线程获取的value:" + threadLocal.get()); Thread thread = new Thread(new Runnable() { @Override public void run() { String value = threadLocal.get(); System.out.println("子线程获取的value:" + value); // 一定要remove,不然可能导致内存泄漏 threadLocal.remove(); } }); thread.start(); } }
结果(子线程无法获取父线程设置的值)
主线程获取的value:hello
子线程获取的value:null
解决方案
只需要将ThreadLocal变成InheritableThreadLocal。
代码
package com.example.a; public class Demo { private static InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>(); public static void main(String[] args) { inheritableThreadLocal.set("hello"); System.out.println("主线程获取的value:" + inheritableThreadLocal.get()); Thread thread = new Thread(new Runnable() { @Override public void run() { String value = inheritableThreadLocal.get(); System.out.println("子线程获取的value:" + value); // 一定要remove,不然可能导致内存泄漏 inheritableThreadLocal.remove(); } }); thread.start(); } }
结果(子线程可以获取父线程设置的值)
主线程获取的value:hello
子线程获取的value:hello
源码分析
源码查看
InheritableThreadLocal的源代码:
package java.lang; import java.lang.ref.*; 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); } }
这个类继承了ThreadLocal,并且重写了getMap和createMap方法,区别是:InheritableThreadLocal将 ThreadLocal 中的 threadLocals 换成了 inheritableThreadLocals,这两个变量都是ThreadLocalMap类型,并且都是Thread类的属性。
InheritableThreadLocal为什么能拿到父线程中的ThreadLocal值?
1.InheritableThreadLocal的get方法
InheritableThreadLocal获取值先调用了get方法,所以我们直接看看get方法都做了些啥。
public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } return setInitialValue(); }
可以看出,get方法和ThreadLocal中是一样的,唯一有区别的就是其中的getMap方法重写了,返回的是inheritableThreadLocals属性。这个属性也是一个ThreadLocalMap类型的变量。那么可以推断:是在某处将父线程中的ThreadLocal值赋值到了子线程的inheritableThreadLocals中。
2.子线程inheritableThreadLocals的赋值过程
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(); 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; this.daemon = parent.isDaemon(); this.priority = parent.getPriority(); if (security == null || isCCLOverridden(parent.getClass())) this.contextClassLoader = parent.getContextClassLoader(); else this.contextClassLoader = parent.contextClassLoader; this.inheritedAccessControlContext = acc != null ? acc : AccessController.getContext(); this.target = target; setPriority(priority); //1. 这边先判断了父线程中inheritableThreadLocals属性是否为空,不为空的话就复制给子线程 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(); }
注意
一旦子线程被创建以后,再操作父线程中的ThreadLocal变量,那么子线程是不能感知的。因为父线程和子线程还是拥有各自的ThreadLocalMap,只是在创建子线程的“一刹那”将父线程的ThreadLocalMap复制给子线程,后续两者就没啥关系了。
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