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Java多线程–InheritableThreadLocal的用法(有实例)

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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