Java中的ThreadLocal线程池原理

目录

• ThreadLocal
• set方法：
• get方法
• ThreadLocalMap
• ThreadLocal弱引用

ThreadLocal

ThreadLocal提供了线程的局部变量（或本地变量）。

它可以保证访问到的变量属于当前线程，每个访问这种变量的线程（通过它的get或set方法）都有自己的、独立初始化的变量副本，每个线程的变量都不同。

ThreadLocal相当于提供了一种线程隔离，将变量与线程相绑定。

ThreadLocal类定义如下：可以简单瞄一眼吆，毕竟没有浏览的欲望....

public class ThreadLocal<T> {
   
    private final int threadLocalHashCode = nextHashCode();
    private static AtomicInteger nextHashCode = new AtomicInteger();
    private static final int HASH_INCREMENT = 0x61c88647;
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }
 
    protected T initialValue() {
        return null;
    }
 
    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }
 
    public ThreadLocal() {
    }
 
    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();
    }
 
    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;
    }
 
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
 
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }
 
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
 
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
 
    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }
 
    T childValue(T parentValue) {
        throw new UnsupportedOperationException();
    }
 
    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
 
        private final Supplier<? extends T> supplier;
 
        SuppliedThreadLocal(Supplier<? extends T> supplier) {
            this.supplier = Objects.requireNonNull(supplier);
        }
 
        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }
 
    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 Entry[] table;
 
       
        private int size = 0;
 
       
        private int threshold; // Default to 0
 
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }
 
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }
 
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
 
        ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
 
        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) {
                    @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++;
                    }
                }
            }
        }
 
        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
                return getEntryAfterMiss(key, i, e);
        }
 
        private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
            Entry[] tab = table;
            int len = tab.length;
 
            while (e != null) {
                ThreadLocal<?> k = e.get();
                if (编程客栈k == key)
                    return e;
                if (k == null)
                    expungeStaleEntry(i);
                else
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }
 
        private void set(ThreadLocal<?> key, Object value) {
 
            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);
 
            for (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 Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }
 
       
        private void remove(ThreadLocal<?> key) {
            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }
 
       
        private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;
            int slotToExpunge = staleSlot;
            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;
 
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal<?> k = e.get();
 
                if (k == key) {
                    e.value = value;
 
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;
 
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }
 
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }
 
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);
 
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }
 
        private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
 
            tab[staleSlot].value = null;
            tab[staleSlot] = null;
            size--;
 
            Entry e;
            int i;
            for (i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal<?> k = e.get();
                if (k == null) {
                    e.value = null;
                    tab[i] = null;
                    size--;
                } else {
                    int h = k.threadLocalHashCode & (len - 1);
                    if (h != i) {
                        tab[i] = null;
 
                        while (tab[h] != null)
                            h = nextIndex(h, len);
                        tab[h] = e;
                    }
                }
            }
            return i;
        }
 
        private boolean cleanSomeSlots(int i, int n) {
            boolean removed = false;
            Enpythontry[] tab = table;
            int len = tab.length;
            do {
                i = nextIndex(i, len);
                Entry e = tab[i];
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }
 
        private void rehash() {
            expungeStaleEntries();
 
            if (size >= threshold - threshold / 4)
                resize();
        }
 
        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;
        }
 
        private void expungeStaleEntries() {
            Entry[] tab = table;
            int len = tab.length;
            for (int j = 0; j < len; j++) {
                Entry e = tab[j];
                if (e != null && e.get() == null)
                    expungeStaleEntry(j);
            }
        }
    }
}

 内部方法：

ThreadLocal通过threadLocalHashCode来标识每一个ThreadLocal的唯一性。threadLocalHashCode通过CAS操作进行更新，每次hash操作的增量为 0x61c88647（不知为何）。

接下来看下ThreadLocal的set、get等相关主要方法

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);
    }

通过Thread.currentThread()方法获取了当前的线程引用，并传给了getMap(Thread)方法获取一个ThreadLocalMap的实例。我们继续跟进getMap(Thread)方法：

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    python}

可以看到getMap(Thread)方法直接返回Thread实例的成员变量threadLocals。它的定义在Thread内部，访问级别为package级别：

public class Thread implements Runnable {
    private static native void registerNatives();
    static {
        registerNatives();
    }
 
    private volatile char  name[];
    private int            priority;
    private Thread         threadQ;
    private long           eetop;
    private boolean     single_step;
    private boolean     daemon = false;
    private boolean     stillborn = false;
    private Runnable target;
    private ThreadGroup group;
    private ClassLoader contextClassLoader;
    private AccessControlContext inheritedAccessControlContext;
    private static int threadInitNumber;
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }
 
    ThreadLocal.ThreadLocalMap threadLocals = null;
 
    ..........
 
}

到了这里，我们可以看出，每个Thread里面都有一个ThreadLocal.ThreadLocalMap成员变量，也就是说每个线程通过ThreadLocal.ThreadLocalMap与ThreadLocal相绑定，这样可以确保每个线程访问到的thread-local variable都是本线程的。

我们往下继续分析。获取了ThreadLocalMap实例以后，如果它不为空则调用ThreadLocalMap.ThreadLocalMap 的set方法设值；若为空则调用ThreadLocal 的createMap方法new一个ThreadLocalMap实例并赋给Thread.threadLocals。

ThreadLocal 的 createMap方法的源码如下：

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

而ThreadLocalMap是ThreadLocal的一个静态内部类，在文章开头贴出的ThreadLocal源码可查看。

总结：

set操作是向当前线程的ThreadLocal.ThreadLocalMap类型的成员变量threadLocals中设置值，key是this，value是我们指定的值

注意，这里传的this代表的是那个ThreadLocal类型的变量（或者说叫对象）

也就是说，每个线程都维护了一个ThreadLocal.ThreadLocalMap类型的对象，而set操作其实就是以ThreadLocal变量为key，以我们指定的值为value，最后将这个键值对封装成Entry对象放到该线程的ThreadLocal.ThreadLocalMap对象中。每个ThreadLocal变量在该线程中都是ThreadLocal.ThreadLocalMap对象中的一个Entry。既然每个ThreadLocal变量都对应ThreadLocal.ThreadLocalMap中的一个元素，那么就可以对这些元素进行读写删除操作。

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();
    }

通过Thread.currentThread()方法获取了当前的线程引用，并传给了getMap(Thread)方法获取一个ThreadLocalMap的实例，getMap方法前面已经贴出来了。继续跟进setInitialValue()方法：

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;
    }

首先调用 initialValue()方法来初始化，然后 通过Thread.currentThread()方法获取了当前的线程引用，并传给了getMap(Thread)方法获取一个ThreadLocalMap的实例，并将 初始化值存到ThreadLocalMap 中。

initialValue() 源码如下：

protected T initialValue() {
        return null;
    }

总结：

get()方法就是从当前线程的ThreadLocal.ThreadLocalMap对象中取出对应的ThreadLocal变量所对应的值

同理，remove()方法就是清除这个值

 public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

ThreadLocal的图形理解：

 或者

 ThreadLocal的使用场景是在线程的声明周期内传值（数据库连接、session管理等），ThreadLocal关键点是在于ThreadLocalMap,可以说一切归功于此，看完上面的描述，应该会有一个直观的体会吧。

下面我们探究一下ThreadLocalMap的实现。

ThreadLocalMap

ThreadLocalMap是ThreadLocal的静态内部类，部分源码如下：

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
 
        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
    }
    
}

其中INITIAL_CAPACITY代表这个Map的初始容量；1是一个Entry类型的数组，用于存储数据；size代表表中的存储数目；threshold代表需要扩容时对应size的阈值。

Entry类是ThreadLocalMap的静态内部类，用于存储数据。

Entry类继承了WeakReference<ThreadLocal<?>>，即每个Entry对象都有一个ThreadLocal的弱引用（作为key），这是为了防止内存泄露。一旦线程结束，key变为一个不可达的对象，这个Entry就可以被GC了。

接下来我们来看ThreadLocalMap 的set方法的实现：

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 exi编程客栈sting 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);
 
            for (Entry e = tab[i];
                 e != null;
   js              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 Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

ThreadLocal 的get方法会调用 ThreadLocalMap 的 getEntry(ThreadLocal key) ，其源码如下：

 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
            return getEntryAfterMiss(key, i, e);
    }
 
    private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
        Entry[] tab = table;
        int len = tab.length;
 
        while (e != null) {
            ThreadLocal k = e.get();
            if (k == key)
                return e;
            if (k == null)
                expungeStaleEntry(i);
            else
                i = nextIndex(i, len);
            e = tab[i];
        }
        return null;
    }

ThreadLocal弱引用

说ThreadLocal是一个弱引用，其本质是ThreadLocal类中ThreadLocalMap类中的Entry的key是一个弱引用。前面提到过Entry中的key是this，this指向ThreadLocal

在ThreadLocal源码中，截取一段ThreadLocalMap的源码如下：

static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;
 
            Entry(ThreadLocal<?> k, Object v) {
                //由于Entry继承了WeakReference，所以这里以一个弱引用指向ThreadLcoal对象
                super(k);
                value = v;
            }
        }

为什么要这么做呢？

看下面的这种场景：

public void func1() {
        ThreadLocal tl = new ThreadLocal<Integer>(); //line1
         tl.set(100);   //line2
         tl.get();       //line3
}

line1新建了一个ThreadLocal对象，t1 是强引用指向这个对象；line2调用set（）后，新建一个Entry，通过源码可知entry对象里的 k是弱引用指向这个对象。如图：

当func1方法执行完毕后，栈帧销毁，强引用 tl 也就没有了，但此时线程的ThreadLocalMap里某个entry的 k 引用还指向这个对象。若这个k 引用是强引用，就会导致k指向的ThreadLocal对象及v指向的对象不能被gc回收，造成内存泄漏，但是弱引用就不会有这个问题（弱引用及强引用等这里不说了）。使用弱引用，就可以使ThreadLocal对象在方法执行完毕后顺利被回收，而且在entry的k引用为null后，再调用get,set或remove方法时，就会尝试删除key为null的entry，可以释放value对象所占用的内存。

概括说就是：在方法中新建一个ThreadLocal对象，就有一个强引用指向它，在调用set（）后，线程的ThreadLocalMap对象里的Entry对象又有一个引用 k 指向它。如果后面这个引用 k 是强引用就会使方法执行完，栈帧中的强引用销毁了，对象还不能回收，造成严重的内存泄露。

注意：虽然弱引用，保证了k指向的ThreadLocal对象能被及时回收，但是v指向的value对象是需要ThreadLocalMap调用get、set时发现k为null时才会去回收整个entry、value，因此弱引用不能保证内存完全不泄露。我们要在不使用某个ThreadLocal对象后，手动调用remoev方法来删除它，尤其是在线程池中，不仅仅是内存泄露的问题，因为线程池中的线程是重复使用的，意味着这个线程的ThreadLocalMap对象也是重复使用的，如果我们不手动调用remove方法，那么后面的线程就有可能获取到上个线程遗留下来的value值，造成bug。

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