ThreadLocal和InheritableThreadLocal详解

2023年 10月 7日 42.7k 0

一、概述

为了方便且更加安全的进行多线程编程,jdk引入ThreadLocal和InheritableThreadLocal两个类,以供开发人员进行多线程之间的数据传递和数据共享。InheritableThreadLocal是ThreadLocal的子类,它可以实现子线程共享父线程的变量。

二、案例介绍

ThreadLocal:

private static ThreadLocal testThreadLocal = new ThreadLocal();
/*
// 创建时可重写初始化方法
ThreadLocal testThreadLocal = new ThreadLocal(){
    public Connection initialValue(){
        return "zhangsan";
    }
};
*/

public static void main(String[] args) {
        // 设置线程变量
        testThreadLocal.set("zhangsan");
        // 获取线程变量
        String userName = testThreadLocal.get();
        System.out.println("userName: " + userName);
        // 删除线程变量
        testThreadLocal.remove();
        userName = testThreadLocal.get();
        System.out.println("userName: " + userName);
}
#结果输出
userName: zhangsan
userName: null
public static void main(String[] args) {
        // 主线程
        testThreadLocal.set("zhangsan");
        System.out.println("userName0: " + testThreadLocal.get());
        // 线程1
        new Thread(() -> {
            testThreadLocal.set("lisi");
            System.out.println("userName1: " + testThreadLocal.get());
        }).start();

        // 线程2
        new Thread(() -> {
            testThreadLocal.set("wangwu");
            System.out.println("userName2: " + testThreadLocal.get());
        }).start();
}
#结果输出【线程之间变量相关隔离】
userName0: zhangsan
userName1: lisi
userName2: wangwu

接下来看下set方法:

public void set(T value) {
    // 获取当前线程(调用方线程:主线程、线程1......)
    Thread t = Thread.currentThread();
    // 当前线程作为key,获取对应的线程变量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 设置线程变量:key为当前定义的ThreadLocal实例的this引用,值为我们传入的数据
        map.set(this, value);
    } else {
        // 第一次设置线程变量,则会创建ThreadLocalMap
        createMap(t, value);
    }    
}

再看下get方法:

public T get() {
    // 获取当前线程(调用方线程:主线程、线程1......)
    Thread t = Thread.currentThread();
    // 当前线程作为key,获取对应的线程变量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 值最终是存在Entry对象的value属性
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            T result = (T)e.value;
            return result;
        }
    }
    // ThreadLocalMap为空,则初始化操作
    return setInitialValue();
}
 
private T setInitialValue() {
    // 创建ThreadLocal时可重写初始化方法
    T value = initialValue();
    // 获取当前线程(调用方线程:主线程、线程1......)
    Thread t = Thread.currentThread();
    // 当前线程作为key,获取对应的线程变量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 设置线程变量:key为当前定义的ThreadLocal实例的this引用,值为初始化方法返回的数据
        map.set(this, value);
     } else {
        // 第一次设置线程变量,则会创建ThreadLocalMap
        createMap(t, value);
     }    
    return value;
}

InheritableThreadLocal:

private static InheritableThreadLocal testInheritableThreadLocal = new InheritableThreadLocal();

    public static void main(String[] args) {
        // 主线程
        testInheritableThreadLocal.set("zhangsan");
        System.out.println("userName0: " + testInheritableThreadLocal.get());
        
        // 线程1
        new Thread(() -> System.out.println("userName1: " + testInheritableThreadLocal.get())).start();

        // 线程2
        new Thread(() -> System.out.println("userName2: " + testInheritableThreadLocal.get())).start();
    }
#结果输出
userName0: zhangsan
userName1: zhangsan
userName2: zhangsan

查看InheritableThreadLocal的源码:

public class InheritableThreadLocal extends ThreadLocal {
 
    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);
    }
}

InheritableThreadLocal继承了ThreadLocal类型,并且重写了getMap和createMap方法,唯一的区别是:threadLocals(ThreadLocalMap类型)变成了inheritableThreadLocals(ThreadLocalMap类型)。

查看get方法:

public T get() {
        Thread t = Thread.currentThread();
        // 注意:InheritableThreadLocal重写了getMap方法,返回inheritableThreadLocals
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

查看inheritableThreadLocals设置的地方,最终定位到java.lang.Thread#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();
        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);
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
           // 将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals
           // 此处可联想到:如果使用了线程池,而线程池中的线程是复用的,不会再次调用初始化方法
           // 所以无法将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals
            this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        this.stackSize = stackSize;
        tid = nextThreadID();
    }

三、注意事项

  • 使用ThreadLocal、或者InheritableThreadLocal方法时,注意及时调用remove方法进行清理。
  • 注意线程池结合InheritableThreadLocal的使用,线程池中的线程是复用的,不会再次调用初始化方法,所以无法将父线程inheritableThreadLocals复制给子线程inheritableThreadLocals。

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