ThreadLocal

1、数据结构

ThreadLocal对象可以提供线程局部变量，每个线程Thread拥有一份自己的副本变量，多个线程互不干扰。

数据结构如下：

•  Thread中存在threadLocals变量，类型是ThreadLocal.ThreadLocalMap

ThreadLocal.ThreadLocalMap threadLocals = null;

• ThreadLocalMap中存在Entry，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;
            }
        }

        /**
         * 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;

 2、set()方法

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

重点看下 map.set(this, value);

private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1); //获取下标
　　　　　　　 // 如果tab[i]不为空，那么获取它的下一个　　　　　
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
　　　　　　　　　　//如果key相同，覆盖
                if (k == key) {
                    e.value = value;
                    return;
                }
　　　　　　　　　 //如果k == null ，说明Entry是过期数据，进行replaceStaleEntry
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
　　　　　　　　
            tab[i] = new Entry(key, value);
            int sz = ++size;
　　　　　　　 //看情况是否扩容
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

•  threadLocalHashCode中涉及一个HASH_INCREMENT，它的黄金分割数。hash增量为这个数字，带来的好处就是hash分布非常的均匀

    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;

• replaceStaleEntry操作

private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            int slotToExpunge = staleSlot;
　　　　　　　//staleSlot向前查找，当Entry == null则停止，如果找到过期数据，更新soltToExpunge = i

            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever
            // occurs first
　　　　　　　// staleSlot向后查找，当Entry == null停止
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal<?> k = e.get();

                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
　　　　　　　　　　//替换逻辑
                if (k == key) {
                    e.value = value;

                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // Start expunge at preceding stale entry if it exists 
　　　　　　　　　　　　//如果之前slotToExpunge没有改变即没有找到过期数据
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }

                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            // If key not found, put new entry in stale slot 如果没有找到key相同的，那么在staleSlot位置添加元素
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them 不相同证明存在过期数据，进行清理
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }

如果执行完启发式清理工作后，未清理到任何数据，且当前散列数组中Entry的数量已经达到了列表的扩容阈值(len*2/3)，就开始执行rehash()

if (!cleanSomeSlots(i, sz) && sz >= threshold)  rehash();

首先是会进行探测式清理工作,清理完成之后，table中可能有一些key为null的Entry数据被清理掉，所以此时通过判断size >= threshold - threshold / 4 也就是size >= threshold* 3/4 来决定是否扩容

        private void rehash() {
            expungeStaleEntries();

            // Use lower threshold for doubling to avoid hysteresis
            if (size >= threshold - threshold / 4)
                resize();
        }

扩容，两倍

        /**
         * Double the capacity of the table.
         */
        private void resize() {
            Entry[] oldTab = table;
            int oldLen = oldTab.length;
            int newLen = oldLen * 2;
            Entry[] newTab = new Entry[newLen];
            int count = 0;

            for (int j = 0; j < oldLen; ++j) {
                Entry e = oldTab[j];
                if (e != null) {
                    ThreadLocal<?> k = e.get();
                    if (k == null) {
                        e.value = null; // Help the GC
                    } else {
                        int h = k.threadLocalHashCode & (newLen - 1);
                        while (newTab[h] != null)
                            h = nextIndex(h, newLen);
                        newTab[h] = e;
                        count++;
                    }
                }
            }

            setThreshold(newLen);
            size = count;
            table = newTab;
        }

3、get()方法

    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();
    }

setInitialValue()，简单明了

    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

4、ThreadLocal内存泄漏问题

　  现象：比如在使用线程池的情况下，线程一直存活，Entry的key是弱引用会在GC的时候被回收，而value由于存在线程的强引用，不会被回收。虽然ThreadLocal在get()、set()、remove()中都有堆key == null数据的处理。但是极端情况下还是会出现内存泄漏。

　 解决方法：每次使用完ThreadLocal，都调用它的remove()方法，清除数据。

 5、InheritableThreadLocal

如果线程A创建了B线程，那么B线程就是A线程的子线程。

InheritableThreadLocal可以让子线程获取父线程设置的变量。

public class Test01 {

    private static ThreadLocal threadLocal1 = new InheritableThreadLocal();


    public static void main(String[] args) throws InterruptedException {
        System.out.println("begin");
        threadLocal1.set("jack");
        System.out.println("1====" + threadLocal1.get()); //jack

        Thread thread = new Thread(() -> {
            System.out.println("2====" + threadLocal1.get());  // jack
            threadLocal1.set("rose");
            System.out.println("3====" + threadLocal1.get()); //rose
        });
        thread.start();
        thread.join();

        System.out.println("4====" + threadLocal1.get()); //jack
    }

源码分析

   public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }

走

    /**
     * Initializes a Thread with the current AccessControlContext.
     * @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }

走

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) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        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)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

走

 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

走

        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];
　　　　　　　//复制了parent中的值
            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
                    if (key != null) {
                        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++;
                    }
                }
            }
        }

参考：https://www.cnblogs.com/wang-meng/p/12856648.html