解決子線程無法訪問父線程中通過ThreadLocal設(shè)置的變量問題

更新時間：2024年07月08日 11:46:14 作者：沐雨橙風(fēng)ιε

這篇文章主要介紹了解決子線程無法訪問父線程中通過ThreadLocal設(shè)置的變量問題,具有很好的參考價值,希望對大家有所幫助,如有錯誤或未考慮完全的地方,望不吝賜教

一、引出結(jié)論

學(xué)習(xí)過ThreadLocal的童鞋都知道，在子線程中，是無法訪問父線程通過ThreadLocal設(shè)置的變量的。

package thread;
 
/**
 * @author heyunlin
 * @version 1.0
 */
public class ThreadLocalExample {
 
    public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException {
        ThreadLocal<String> threadLocal = new ThreadLocal<>();
 
        threadLocal.set("hello");
 
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("run()...");
 
                /*
                 * 子線程中無法訪問父線程中設(shè)置的ThreadLocal變量
                 */
                System.out.println(threadLocal.get());
            }
        });
 
        thread.start();
        thread.join();
 
        System.out.println(thread);
        System.out.println(threadLocal.get());
    }
 
}

運行結(jié)果：

InheritableThreadLocal就是為了解決這個不可見問題而生的~

package thread;
 
/**
 * @author heyunlin
 * @version 1.0
 */
public class InheritableThreadLocalExample {
 
    public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException {
        InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
 
        threadLocal.set("hello");
 
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("run()...");
                System.out.println(threadLocal.get());
            }
        });
 
        thread.start();
        thread.join();
 
        System.out.println(thread);
        System.out.println(threadLocal.get());
    }
 
}

運行結(jié)果：

二、分析原因

那么，究竟是什么原因?qū)е碌淖泳€程無法訪問父線程中的ThreadLocal變量呢，接下來研究ThreadLocal這個類的源碼。

先看一下這個ThreadLocal類上的文檔注釋，大概了解ThreadLocal是用來干什么的

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
 *
 * public class ThreadId {
 *     // Atomic integer containing the next thread ID to be assigned
 *     private static final AtomicInteger nextId = new AtomicInteger(0);
 *
 *     // Thread local variable containing each thread's ID
 *     private static final ThreadLocal&lt;Integer&gt; threadId =
 *         new ThreadLocal&lt;Integer&gt;() {
 *             &#64;Override protected Integer initialValue() {
 *                 return nextId.getAndIncrement();
 *         }
 *     };
 *
 *     // Returns the current thread's unique ID, assigning it if necessary
 *     public static int get() {
 *         return threadId.get();
 *     }
 * }
 * </pre>
 * <p>Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the {@code ThreadLocal}
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).
 *
 * @author  Josh Bloch and Doug Lea
 * @since   1.2
 */

1、關(guān)鍵注釋

博主在類的文檔注釋上提取了幾個關(guān)于ThreadLocal的重要的說明

This class provides thread-local variables.

這個類提供了線程本地變量

ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread.

ThreadLocal實例提供了可以關(guān)聯(lián)一個線程的靜態(tài)字段。

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

只要線程是存活狀態(tài)，并且ThreadLocal對象可以訪問，每個線程都擁有一個線程本地變量的副本的引用；

在線程執(zhí)行完方法之后，所有線程本地變量都會被gc（垃圾收集器）回收，除非有其他變量引用了它。

2、研究源碼

經(jīng)過上面博主的大致翻譯，已經(jīng)對ThreadLocal有了一定的了解，接下來深入源碼去學(xué)習(xí)ThreadLocal的工作原理。

在不了解ThreadLocal的情況下，直接從我們直接調(diào)用的get()和set()方法入手。

public class ThreadLocal<T> {
 
    public T get() {
        Thread thread = Thread.currentThread();
        ThreadLocalMap map = getMap(thread);
 
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
 
            if (e != null) {
                T result = (T)e.value;
 
                return result;
            }
        }
 
        return setInitialValue();
    }
 
    public void set(T value) {
        Thread thread = Thread.currentThread();
        ThreadLocalMap map = getMap(thread);
 
        if (map != null) {
            map.set(this, value);
        } else {
            createMap(t, value);
        }
    }
 
}

set()方法

    public void set(T value) {
        // 獲取當(dāng)前線程
        Thread thread = Thread.currentThread();
        // 根據(jù)當(dāng)前線程獲取ThreadLocalMap對象
        ThreadLocalMap map = getMap(thread);
 
        // map不為null，設(shè)置初始值到map中
        if (map != null) {
            map.set(this, value);
        } else {
            // map是null，創(chuàng)建一個map
            createMap(t, value);
        }
    }

getMap()

看樣子和線程類Thead里的一個threadLocals變量有關(guān)。

    ThreadLocal.ThreadLocalMap getMap(Thread thread) {
        // 這個方法就是返回Thread的threadLocals變量的值
        return thread.threadLocals;
    }

ThreadLocalMap是ThreadLocal的一個靜態(tài)內(nèi)部類

public class ThreadLocal<T> {
 
    static class ThreadLocalMap {
    
    }
 
}

set()

ThreadLocal.ThreadLocalMap的set()方法，有點復(fù)雜，類似HashMap的底層源碼實現(xiàn)。

    private void set(ThreadLocal<?> key, Object value) {
        ThreadLocal.ThreadLocalMap.Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);
 
        for (ThreadLocal.ThreadLocalMap.Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
            ThreadLocal<?> k = e.get();
 
            if (k == key) {
                e.value = value;
                
                return;
            }
 
            if (k == null) {
                replaceStaleEntry(key, value, i);
                
                return;
            }
        }
 
        tab[i] = new ThreadLocal.ThreadLocalMap.Entry(key, value);
        int sz = ++size;
        
        if (!cleanSomeSlots(i, sz) && sz >= threshold) {
            rehash();
        }
    }

createMap()

    void createMap(Thread thread, T firstValue) {
        // 初始化線程的threadLocals變量
        thread.threadLocals = new ThreadLocal.ThreadLocalMap(this, firstValue);
    }

set()方法總結(jié)

根據(jù)set()方法的源代碼，set()方法是把值設(shè)置到Thead線程類的threadLocals變量中，這個變量應(yīng)該是一個Map派生類的實例。

get()方法

    public T get() {
        Thread thread = Thread.currentThread(); // 獲取當(dāng)前線程
        
        // 通過getMap()方法獲取一個ThreadLocalMap對象
        // 因為這個類是以Map結(jié)尾的，推測是java.util.Map的派生類
        ThreadLocalMap map = getMap(thread);
 
        // 獲取到的map不為null
        if (map != null) {
            // 獲取Map的Entry
            // 說明我們的推測大概率是正確的，因為HashMap中也有個Enty，而且也有value()方法
            ThreadLocalMap.Entry e = map.getEntry(this);
 
            // 獲取的value不為空，則轉(zhuǎn)為指定的類型T（泛型）直接返回
            if (e != null) {
                T result = (T) e.value;
 
                return result;
            }
        }
 
        // 代碼運行到這里，說明獲取到的map是null
        // 因為前面的if中已經(jīng)通過return結(jié)束方法
        return setInitialValue();
    }

getMap()

這個方法的代碼在前面set()方法里已經(jīng)看了~

createMap()

這個方法的代碼在前面set()方法里已經(jīng)看了~

setInitialValue()

這個方法和set()方法的代碼幾乎一模一樣。知識多了一個返回值~

T value = initialValue();

set()方法的代碼

return value;

    private T setInitialValue() {
        // 通過initialValue()方法獲取一個初始化值
        T value = initialValue();
        
        // 獲取當(dāng)前線程
        Thread thread = Thread.currentThread();
        // 根據(jù)當(dāng)前線程獲取ThreadLocalMap對象
        ThreadLocalMap map = getMap(thread);
 
        // map不為null，設(shè)置初始值到map中
        if (map != null) {
            map.set(this, value);
        } else {
            // map是null，創(chuàng)建一個map
            createMap(thread, value);
        }
 
        // 返回初始值
        return value;
    }

get()方法總結(jié)

綜合上面關(guān)于get()方法的源代碼，get()方法是從Thead線程類的threadLocals變量中獲取數(shù)據(jù)。

ThreadLocal.ThreadLocalMap

下面是ThreadLocalMap類的一些關(guān)鍵的代碼，很顯然，這個內(nèi)部類的結(jié)構(gòu)設(shè)計的和HashMap非常相似，通過一個數(shù)組table保存值，根據(jù)傳入的key進行特定的hash運算得到數(shù)組的下標(biāo)，然后獲取對應(yīng)數(shù)組下標(biāo)的值返回。

public class ThreadLocal<T> {
 
    static class ThreadLocalMap {
 
        static class Entry extends WeakReference<ThreadLocal<?>> {
            Object value;
 
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
 
                value = v;
            }
        }
 
        private static final int INITIAL_CAPACITY = 16;
 
        private ThreadLocal.ThreadLocalMap.Entry[] table;
 
        private int size = 0;
 
        private int threshold;
 
        private ThreadLocal.ThreadLocalMap.Entry getEntry(ThreadLocal<?> key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            ThreadLocal.ThreadLocalMap.Entry e = table[i];
            
            if (e != null && e.get() == key) {
                return e;
            } else {
                return getEntryAfterMiss(key, i, e);
            }
        }
    }
}

三、代碼總結(jié)

文章稍微學(xué)習(xí)的深入了一點，可能有些童鞋已經(jīng)懵了，但是回到set()和get()方法的代碼可以發(fā)現(xiàn)

不管你存儲值的代碼如何復(fù)雜，都是通過當(dāng)前線程作為key存儲在ThreadLocal的靜態(tài)內(nèi)部類ThreadLocalMap中的，知道這點其實就夠了。

在子線程中，由于通過new Thread()新開了一個線程，那么當(dāng)代碼執(zhí)行這個新開的子線程的run()方法內(nèi)部，當(dāng)前線程已經(jīng)不是剛開始執(zhí)行方法的線程了。

比如：在main方法中，執(zhí)行代碼的線程是main線程，也就是主線程，它的線程名就是main。

但是新開一個線程，它的線程名是通過Threa-0開始編號的

通過一個簡單的代碼了解一下

/**
 * @author heyunlin
 * @version 1.0
 */
public class Example {
 
    public static void main(String[] args) {
        System.out.println(Thread.currentThread());
 
        new Thread() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread());
            }
        }.start();
    }
 
}

運行結(jié)果：