深入理解ThreadLocal及其变种

ThreadLocal

定义

ThreadLocal很容易让人望文生义,想当然地认为是一个“本地线程”。

其实,ThreadLocal并不是一个Thread,而是Thread的局部变量,也许把它命名为ThreadLocalVariable更容易让人理解一些。

各个线程的ThreadLocal关联的实例互不干扰。特征:

  • ThreadLocal表示线程的"局部变量",它确保每个线程的ThreadLocal变量都是各自独立的
  • ThreadLocal适合在一个线程的处理流程中保持上下文(避免了同一参数在所有方法中传递)
  • 使用ThreadLocal要用try ... finally结构,并在finally中清除

常用方法

  • set:为当前线程设置变量,当前ThreadLocal作为索引
  • get:获取当前线程变量,当前ThreadLocal作为索引
  • initialValue:(需要子类实现,默认null)执行get时,发现线程本地变量为null,就会执行initialValue的内容
  • remove:清空当前线程的ThreadLocal索引与映射的元素

底层结构及逻辑

Thread对象的属性

public class Thread implements Runnable {
    // .....
    ThreadLocal.ThreadLocalMap threadLocals = null;
    // .....
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
}

ThreadLocalMap对象

public class ThreadLocal<T> {
	// .....
	
    static class ThreadLocalMap {
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }

        /**
         * Increment i modulo len.
         */
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }

        /**
         * Decrement i modulo len.
         */
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
    }    
}

set值流程

源码摘要:

// java.lang.ThreadLocal#set
public void set(T value) {
    Thread t = Thread.currentThread();
    // map惰性创建
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

重点来关注下 java.lang.ThreadLocal.ThreadLocalMap#set 方法

private void set(ThreadLocal<?> key, Object value) {
    Entry[] tab = table;
    int len = tab.length;
    // 根据ThreadLocal对象的hash值,定位到table中的位置i
    int i = key.threadLocalHashCode & (len - 1);
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get();
        
        // 如果位置i不为空,且这个Entry对象的key正好是即将设置的key,那么就覆盖Entry中的value
        if (k == key) {
            e.value = value;
            return;
        }
        
        // 如果当前位置是空的,就初始化一个Entry对象放在位置i上
        if (k == null) {
            // 里面会调到 expungeStaleEntry 
            replaceStaleEntry(key, value, i);
            return;
        }
        
        // 如果位置i的不为空,而且key不等于entry,那就找下一个空位置,直到为空为止
    }
    
    tab[i] = new Entry(key, value);
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
}

结合代码,set的过程如下图

冲突解决

线性探测的方式解决hash冲突的问题,如果没有找到空闲的slot,就不断往后尝试,直到找到一个空闲的位置,插入entry

get流程

源码摘要:

// java.lang.ThreadLocal#get
public T get() {
    Thread t = Thread.currentThread();
    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();// 调用initialValue方法
}
// java.lang.ThreadLocal.ThreadLocalMap#getEntry
private Entry getEntry(ThreadLocal<?> key) {
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        // 可能是没有,或者hash冲突了
        return getEntryAfterMiss(key, i, e);
}

private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
    Entry[] tab = table;
    int len = tab.length;
    // get的时候一样是根据ThreadLocal获取到table的i值,然后查找数据拿到后会对比key是否相等
    while (e != null) {
        ThreadLocal<?> k = e.get();
        // 相等就直接返回,不相等就继续查找,找到相等位置。
        if (k == key)
            return e;
        if (k == null)
            // 清理回收无效value、entry
            expungeStaleEntry(i);
        else
            i = nextIndex(i, len);
        e = tab[i];
    }
    return null;
}

弱引用

static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;

    Entry(ThreadLocal<?> k, Object v) {
        super(k);
        value = v;
    }
}

为什么使用弱引用?

弱引用的特点:弱引用的对象拥有更短暂的生命周期。垃圾回收器线程扫描的时候,一旦发现了具有弱引用的对象,不管当前内存空间足够与否,都会回收它的内存。

结合到这里的场景,当ThreadLocal在没有外部强引用的时候,一旦发生gc,key就会被回收。

内存泄露问题

因为有了弱引用,可以确保Entry的key会被内存回收掉。但是Entry的value和Entry对象本身还是没有得到回收。

如果ThreadLocal的线程一直保持运行,那么这个Entry对象中的value就有可能一直得不到回收,发生内存泄露。

解决办法:在finally里面调用remove方法

扩展

InheritableThreadLocal

InheritableThreadLocal 是 JDK 本身自带的一种线程传递解决方案,以完成父线程到子线程的值传递。在创建子线程的时候,就把父线程的ThreadLocal的内容复制过去。

// java.lang.Thread#init(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String, long, java.security.AccessControlContext, boolean)
private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
    // ...
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        // 复制父线程的InheritableThreadLocal内容
        this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    
    // ...
}


// java.lang.ThreadLocal#createInheritedMap
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    return new ThreadLocalMap(parentMap);
}
private ThreadLocalMap(ThreadLocalMap parentMap) {
    Entry[] parentTable = parentMap.table;
    int len = parentTable.length;
    setThreshold(len);
    table = new Entry[len];
    for (int j = 0; j < len; j++) {
        Entry e = parentTable[j];
        if (e != null) {
            ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
            if (key != null) {
                // 这里的value 是同一个对象
                Object value = key.childValue(e.value);
                Entry c = new Entry(key, value);
                int h = key.threadLocalHashCode & (len - 1);
                while (table[h] != null)
                    h = nextIndex(h, len);
                table[h] = c;
                size++;
            }
        }
    }
}

不过,子线程ThreadLocalMap里的Entry.value指向的对象和父线程是同一个

特殊场景下的缺陷

在线程池的场景下,线程由线程池创建好,并且线程是池化起来反复使用的;这时父子线程关系的ThreadLocal值传递已经没有意义。比如:

public static void main(String[] args) throws InterruptedException {
    // 线程池提前创建好
    ExecutorService executorService = Executors.newFixedThreadPool(2);
    // 提前创建了一个子线程 [pool-1-thread-1]
    executorService.submit(() -> {
        System.out.println(Thread.currentThread().getName());
    });
    Thread.sleep(1000);

    InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal();
    threadLocal.set("start");
    System.out.println(threadLocal.get());

    // 后续,[pool-1-thread-1]线程的ThreadLocal值永远是null
    executorService.submit(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    });
    executorService.submit(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    });
    executorService.submit(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    });

    Thread.sleep(100);
    System.out.println(threadLocal.get());
    executorService.shutdown();
}

// 输出结果
pool-1-thread-1
start
start   ->   pool-1-thread-2
start   ->   pool-1-thread-2
null   ->   pool-1-thread-1
start

尤其是现在都是基于框架开发,线程池一般在项目启动的时候,就创建好了。业务代码提交执行任务的时候,如果复用之前的线程,那么值就没传到子线程中去!

像这种情况,我们至少要求 把任务提交给线程池时 的ThreadLocal值传递到执行线程中。TransmittableThreadLocal的出现就是为了解决这个问题。

TransmittableThreadLocal

TransmittableThreadLocal是Alibaba开源的一个类,它继承了InheritableThreadLocal。能实现在线程池和主线程之间传递,需要配合TtlRunnable 和 TtlCallable使用。

使用示例

public static void main(String[] args) throws InterruptedException {
    // 线程池提前创建好
    ExecutorService executorService = Executors.newFixedThreadPool(2);
    // 提前创建了一个子线程 [pool-1-thread-1]
    executorService.submit(() -> {
        System.out.println(Thread.currentThread().getName());
    });
    Thread.sleep(1000);

    TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal();
    threadLocal.set("start");
    System.out.println(threadLocal.get());

    // 每次提交时都需要通过修饰操作(即TtlRunnable.get(task))以抓取这次提交时的TransmittableThreadLocal上下文的值
    executorService.submit(TtlRunnable.get(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    }));
    executorService.submit(TtlRunnable.get(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    }));
    executorService.submit(TtlRunnable.get(() -> {
        System.out.println(threadLocal.get() + "   ->   " + Thread.currentThread().getName());
    }));

    Thread.sleep(100);
    System.out.println(threadLocal.get());
    executorService.shutdown();
}

// 输出结果
pool-1-thread-1
start
start   ->   pool-1-thread-1
start   ->   pool-1-thread-2
start   ->   pool-1-thread-1
start

整个过程的完整时序图

修饰线程池

使用TTL的时候,每次提交任务时,都需要用TtlRunnable 或者 TtlCallable对任务修饰一下。这个修饰逻辑可以再线程池中完成。

通过工具类com.alibaba.ttl.threadpool.TtlExecutors完成,有下面的方法:

  • getTtlExecutor:修饰接口Executor
  • getTtlExecutorService:修饰接口ExecutorService
  • getTtlScheduledExecutorService:修饰接口ScheduledExecutorService

示例代码:

ExecutorService executorService = ...
// 额外的处理,生成修饰了的对象executorService
executorService = TtlExecutors.getTtlExecutorService(executorService);

TransmittableThreadLocal<String> context = new TransmittableThreadLocal<>();

// =====================================================

// 在父线程中设置
context.set("value-set-in-parent");

Runnable task = new RunnableTask();
Callable call = new CallableTask();
executorService.submit(task);
executorService.submit(call);

// =====================================================

// Task或是Call中可以读取,值是"value-set-in-parent"
String value = context.get();

FastThreadLocal

前面分析了ThreadLocal的get和set,当遇到hash冲突的时候,会以nextIndex计算下一个位置的方式来解决hash冲突。

使用线性探测的方式解决hash冲突的问题,如果没有找到空闲的slot,就不断往后尝试,直到找到一个空闲的位置,插入entry,这种方式在经常遇到hash冲突时,影响效率。

鉴于此,netty提供了FastThreadLocal。与之配套的还有FastThreadLocalThread和FastThreadLocalRunnable。

创建FastThreadLocal对象的时候,直接把位置index(使用AtomicInteger实现)确定下来。每个FastThreadLocal都能获取到一个不重复的下标

public FastThreadLocal() {
    index = InternalThreadLocalMap.nextVariableIndex();
}


public static int nextVariableIndex() {
    int index = nextIndex.getAndIncrement();
    if (index < 0) {
        nextIndex.decrementAndGet();
        throw new IllegalStateException("too many thread-local indexed variables");
    }
    return index;
}

不过,FastThreadLocal需要配合FastThreadLocalThread使用,才能发挥它的效率。

public final void set(V value) {
    if (value != InternalThreadLocalMap.UNSET) {
        InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
        setKnownNotUnset(threadLocalMap, value);
    } else {
        remove();
    }
}
public final V get() {
    InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
    Object v = threadLocalMap.indexedVariable(index);// 直接定位
    if (v != InternalThreadLocalMap.UNSET) {
        return (V) v;
    }
    return initialize(threadLocalMap);
}
public Object indexedVariable(int index) {
    Object[] lookup = indexedVariables;
    return index < lookup.length? lookup[index] : UNSET;
}


// InternalThreadLocalMap.get()
public static InternalThreadLocalMap get() {
    Thread thread = Thread.currentThread();
    // 判断当前Thread类型
    if (thread instanceof FastThreadLocalThread) {
        return fastGet((FastThreadLocalThread) thread);
    } else {
        return slowGet();
    }
}

private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
    // FastThreadLocalThread继承Thread,额外有InternalThreadLocalMap类型属性
    InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
    if (threadLocalMap == null) {
        thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
    }
    return threadLocalMap;
}

private static InternalThreadLocalMap slowGet() {
    // 普通Thread无InternalThreadLocalMap,但有ThreadLocal属性,在它里面存InternalThreadLocalMap等于间接有了InternalThreadLocalMap
    ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = UnpaddedInternalThreadLocalMap.slowThreadLocalMap;
    InternalThreadLocalMap ret = slowThreadLocalMap.get();
    if (ret == null) {
        ret = new InternalThreadLocalMap();
        slowThreadLocalMap.set(ret);
    }
    return ret;
}

也就是说,如果是普通Thread使用FastThreadLocal,则需要先拿到ThreadLocal对象,然后再get到里面存的InternalThreadLocalMap。这一get过程完全是ThreadLocal的get,也需要执行hash碰撞&getEntryAfterMiss等逻辑。(有的地方称之为退化

posted @ 2022-03-04 15:38  yejg1212  阅读(505)  评论(2编辑  收藏  举报