一、概述

為了方便且更加安全的進(jìn)行多線(xiàn)程編程，jdk引入ThreadLocal和InheritableThreadLocal兩個(gè)類(lèi)，以供開(kāi)發(fā)人員進(jìn)行多線(xiàn)程之間的數(shù)據(jù)傳遞和數(shù)據(jù)共享。InheritableThreadLocal是ThreadLocal的子類(lèi)，它可以實(shí)現(xiàn)子線(xiàn)程共享父線(xiàn)程的變量。

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二、案例介紹

ThreadLocal：

private static ThreadLocal testThreadLocal = new ThreadLocal<>();
/*
// 創(chuàng)建時(shí)可重寫(xiě)初始化方法
ThreadLocal testThreadLocal = new ThreadLocal(){
    public Connection initialValue(){
        return "zhangsan";
    }
};
*/

public static void main(String[] args) {
        // 設(shè)置線(xiàn)程變量
        testThreadLocal.set("zhangsan");
        // 獲取線(xiàn)程變量
        String userName = testThreadLocal.get();
        System.out.println("userName: " + userName);
        // 刪除線(xiàn)程變量
        testThreadLocal.remove();
        userName = testThreadLocal.get();
        System.out.println("userName: " + userName);
}
#結(jié)果輸出
userName: zhangsan
userName: null

public static void main(String[] args) {
        // 主線(xiàn)程
        testThreadLocal.set("zhangsan");
        System.out.println("userName0: " + testThreadLocal.get());
        // 線(xiàn)程1
        new Thread(() -> {
            testThreadLocal.set("lisi");
            System.out.println("userName1: " + testThreadLocal.get());
        }).start();

        // 線(xiàn)程2
        new Thread(() -> {
            testThreadLocal.set("wangwu");
            System.out.println("userName2: " + testThreadLocal.get());
        }).start();
}
#結(jié)果輸出【線(xiàn)程之間變量相關(guān)隔離】
userName0: zhangsan
userName1: lisi
userName2: wangwu

接下來(lái)看下set方法：

public void set(T value) {
    // 獲取當(dāng)前線(xiàn)程(調(diào)用方線(xiàn)程：主線(xiàn)程、線(xiàn)程1......)
    Thread t = Thread.currentThread();
    // 當(dāng)前線(xiàn)程作為key，獲取對(duì)應(yīng)的線(xiàn)程變量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 設(shè)置線(xiàn)程變量：key為當(dāng)前定義的ThreadLocal實(shí)例的this引用，值為我們傳入的數(shù)據(jù)
        map.set(this, value);
    } else {
        // 第一次設(shè)置線(xiàn)程變量，則會(huì)創(chuàng)建ThreadLocalMap
        createMap(t, value);
    }    
}

再看下get方法：

public T get() {
    // 獲取當(dāng)前線(xiàn)程(調(diào)用方線(xiàn)程：主線(xiàn)程、線(xiàn)程1......)
    Thread t = Thread.currentThread();
    // 當(dāng)前線(xiàn)程作為key，獲取對(duì)應(yīng)的線(xiàn)程變量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 值最終是存在Entry對(duì)象的value屬性
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            T result = (T)e.value;
            return result;
        }
    }
    // ThreadLocalMap為空，則初始化操作
    return setInitialValue();
}
 
private T setInitialValue() {
    // 創(chuàng)建ThreadLocal時(shí)可重寫(xiě)初始化方法
    T value = initialValue();
    // 獲取當(dāng)前線(xiàn)程(調(diào)用方線(xiàn)程：主線(xiàn)程、線(xiàn)程1......)
    Thread t = Thread.currentThread();
    // 當(dāng)前線(xiàn)程作為key，獲取對(duì)應(yīng)的線(xiàn)程變量ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        // 設(shè)置線(xiàn)程變量：key為當(dāng)前定義的ThreadLocal實(shí)例的this引用，值為初始化方法返回的數(shù)據(jù)
        map.set(this, value);
     } else {
        // 第一次設(shè)置線(xiàn)程變量，則會(huì)創(chuàng)建ThreadLocalMap
        createMap(t, value);
     }    
    return value;
}

InheritableThreadLocal：

private static InheritableThreadLocal testInheritableThreadLocal = new InheritableThreadLocal<>();

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

        // 線(xiàn)程2
        new Thread(() -> System.out.println("userName2: " + testInheritableThreadLocal.get())).start();
    }
#結(jié)果輸出
userName0: zhangsan
userName1: zhangsan
userName2: zhangsan

查看InheritableThreadLocal的源碼：

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繼承了ThreadLocal類(lèi)型，并且重寫(xiě)了getMap和createMap方法，唯一的區(qū)別是：threadLocals(ThreadLocalMap類(lèi)型)變成了inheritableThreadLocals(ThreadLocalMap類(lèi)型)。

查看get方法：

public T get() {
        Thread t = Thread.currentThread();
        // 注意：InheritableThreadLocal重寫(xiě)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設(shè)置的地方，最終定位到j(luò)ava.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)
           // 將父線(xiàn)程inheritableThreadLocals復(fù)制給子線(xiàn)程inheritableThreadLocals
           // 此處可聯(lián)想到：如果使用了線(xiàn)程池，而線(xiàn)程池中的線(xiàn)程是復(fù)用的，不會(huì)再次調(diào)用初始化方法
           // 所以無(wú)法將父線(xiàn)程inheritableThreadLocals復(fù)制給子線(xiàn)程inheritableThreadLocals
            this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        this.stackSize = stackSize;
        tid = nextThreadID();
    }

三、注意事項(xiàng)

• 使用ThreadLocal、或者InheritableThreadLocal方法時(shí)，注意及時(shí)調(diào)用remove方法進(jìn)行清理。
• 注意線(xiàn)程池結(jié)合InheritableThreadLocal的使用，線(xiàn)程池中的線(xiàn)程是復(fù)用的，不會(huì)再次調(diào)用初始化方法，所以無(wú)法將父線(xiàn)程inheritableThreadLocals復(fù)制給子線(xiàn)程inheritableThreadLocals。

分享題目：ThreadLocal和InheritableThreadLocal詳解
文章來(lái)源：http://uogjgqi.cn/article/dphojcg.html