ThreadLocal源码分析（Android-30）

如果做到线程分离？

为什么会导致内存泄漏？

为什么这么设计？

如何解决的？

/*

• Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
• DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
• This code is free software; you can redistribute it and/or modify it
• under the terms of the GNU General Public License version 2 only, as
• published by the Free Software Foundation. Oracle designates this
• particular file as subject to the "Classpath" exception as provided
• by Oracle in the LICENSE file that accompanied this code.
• This code is distributed in the hope that it will be useful, but WITHOUT
• ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
• FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
• version 2 for more details (a copy is included in the LICENSE file that
• accompanied this code).
• You should have received a copy of the GNU General Public License version
• 2 along with this work; if not, write to the Free Software Foundation,
• Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
• Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
• or visit www.oracle.com if you need additional information or have any
• questions. */

package java.lang; import java.lang.ref.*; import java.util.Objects; import java.util.concurrent.atomic.AtomicInteger; import java.util.function.Supplier;

public class ThreadLocal {

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() {
  //构造方法，什么都没做
}

public T get() {
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);//获取该线程独有的成员ThreadLocal.ThreadLocalMap
    if (map != null) {//命中
        //从Entry[]数组table中根据hashCode值获取Entry
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();//创建该线程的ThreadLocal.ThreadLocalMap
}

private T setInitialValue() {
    T value = initialValue();//initialValue()这个方法的返回null
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);//这里会new ThreadLocalMap(this, value);
    return value;
}


public void set(T value) {
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);//获取该线程独有的成员ThreadLocal.ThreadLocalMap
    if (map != null)
        map.set(this, value);//注意这里，插入时会做优化操作，尽可能的去避免内存泄漏
    else
        createMap(t, value);//这里会new ThreadLocalMap(this, 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 {
  //注意这里，Entry继承WeakReference，垃圾回收时，可能会回收掉Entry的key，导致内存泄漏
    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.
     */
  //ThreadLocal.ThreadLocalMap的初始容量 
  //必须为2的幂，具体为什么，这里大概提一下。
  //因为2的幂次-1，转为二进制位全为1，例如；7:111，15:1111
  //在放入table[]时，通过hashCode&（2的幂-1）， 可以尽可能的不重复和均匀插入
    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.
     */
    //获取下一个index,如超过len，从头开始
    private static int nextIndex(int i, int len) {
        return ((i + 1 < len) ? i + 1 : 0);
    }

    /**
     * Decrement i modulo len.
     */
    //获取上一个index,如小于0，从最后开始
    private static int prevIndex(int i, int len) {
        return ((i - 1 >= 0) ? i - 1 : len - 1);
    }

    /**
     * Construct a new map initially containing (firstKey, firstValue).
     * ThreadLocalMaps are constructed lazily, so we only create
     * one when we have at least one entry to put in it.
     */
    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
        table = new Entry[INITIAL_CAPACITY];
        //通过计算，获取插入的index位置
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);//设置阈值
    }

    /**
     * Construct a new map including all Inheritable ThreadLocals
     * from given parent map. Called only by createInheritedMap.
     *
     * @param parentMap the map associated with parent thread.
     */
    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++;
                }
            }
        }
    }

    /**
     * Get the entry associated with key.  This method
     * itself handles only the fast path: a direct hit of existing
     * key. It otherwise relays to getEntryAfterMiss.  This is
     * designed to maximize performance for direct hits, in part
     * by making this method readily inlinable.
     *
     * @param  key the thread local object
     * @return the entry associated with key, or null if no such
     */
    private Entry getEntry(ThreadLocal<?> key) {
        int i = key.threadLocalHashCode & (table.length - 1);//hash获取index
        Entry e = table[i];
        if (e != null && e.get() == key)//命中
            return e;
        else//hash得到到index没有值，或者key==null（被垃圾回收掉）
            return getEntryAfterMiss(key, i, e);
    }

    /**
     * Version of getEntry method for use when key is not found in
     * its direct hash slot.
     *
     * @param  key the thread local object
     * @param  i the table index for key's hash code
     * @param  e the entry at table[i]
     * @return the entry associated with key, or null if no such
     */
    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);//删除
            else
                i = nextIndex(i, len);
            e = tab[i];
        }
        return null;
    }

    /**
     * Set the value associated with key.
     *
     * @param key the thread local object
     * @param value the value to be set
     */
    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;//获取当前Entry[]
        int len = tab.length;//拿到长度
        int i = key.threadLocalHashCode & (len-1);//计算出插入的index

        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            ThreadLocal<?> k = e.get();

            if (k == key) {//i上有key，且相等
                e.value = value;//直接覆盖原来的值
                return;
            }
          
          //注意这里，e不为null，但是key为null
          //因为Entry的key为weakReference弱引用
          //弱引用在垃圾回收时，只要扫描到，就会回收
          
          
            if (k == null) {//这里对内存泄漏做了优化
                replaceStaleEntry(key, value, i);
                return;
            }
        }

        tab[i] = new Entry(key, value);
        int sz = ++size;
        if (!cleanSomeSlots(i, sz) && sz >= threshold)
            rehash();
    }

    /**
     * Remove the entry for key.
     */
    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;

        // 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;//需要清除的index
        //从后往前轮询
        for (int i = prevIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = prevIndex(i, len))
            if (e.get() == null)//找到key被回收的Entry
                slotToExpunge = i;

        // Find either the key or trailing null slot of run, whichever
        // occurs first
        //从前往后轮询
        for (int i = nextIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = nextIndex(i, len)) {
            ThreadLocal<?> k = e.get();
            if (k == key) {
                e.value = value;//value覆盖

                tab[i] = tab[staleSlot];//把被回收的entry赋值给下标为i的entry
                tab[staleSlot] = e;//赋值给原被回收的entry位置

                // Start expunge at preceding stale entry if it exists
                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
        tab[staleSlot].value = null;
        tab[staleSlot] = new Entry(key, value);

        // If there are any other stale entries in run, expunge them
        if (slotToExpunge != staleSlot)
            cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
    }

    //删除已被回收的entry
    private int expungeStaleEntry(int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;

        // expunge entry at staleSlot
        tab[staleSlot].value = null;
        tab[staleSlot] = null;
        size--;

        // Rehash until we encounter null
        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 {
              //从新hash，获取插入table的index
                int h = k.threadLocalHashCode & (len - 1);
                if (h != i) {//位置有变化
                    tab[i] = null;//原位置赋null

                    // Unlike Knuth 6.4 Algorithm R, we must scan until
                    // null because multiple entries could have been stale.
                    while (tab[h] != null)//当前位置有值
                        h = nextIndex(h, len);//往后查找，直到该位置为null
                    tab[h] = e;
                }
            }
        }
        return i;
    }

  //清理
    private boolean cleanSomeSlots(int i, int n) {
        boolean removed = false;
        Entry[] tab = table;
        int len = tab.length;
        do {
            i = nextIndex(i, len);
            Entry e = tab[i];
            if (e != null && e.get() == null) {//被回收的entry
                n = len;
                removed = true;
                i = expungeStaleEntry(i);
            }
        } while ( (n >>>= 1) != 0);
        return removed;
    }

    /**
     * Re-pack and/or re-size the table. First scan the entire
     * table removing stale entries. If this doesn't sufficiently
     * shrink the size of the table, double the table size.
     */
    private void rehash() {
        expungeStaleEntries();

        // Use lower threshold for doubling to avoid hysteresis
        if (size >= threshold - threshold / 4)//大于等于阈值的3/4
            resize();
    }

    /**
     * Double the capacity of the table.
     */
  //扩容
    private void resize() {
      //新table长度为旧table的两倍
        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) {
                  //key为空，value也赋null，使其可以被gc扫描回收
                    e.value = null; // Help the GC
                } else {
                    int h = k.threadLocalHashCode & (newLen - 1);//通过新的table，获取hash值
                    while (newTab[h] != null)//依次往后查询获取entry==null的位置
                        h = nextIndex(h, newLen);
                    newTab[h] = e;
                    count++;
                }
            }
        }

      //重设阈值、大小以及table
        setThreshold(newLen);
        size = count;
        table = newTab;
    }

    /**
     * Expunge all stale entries in the table.
     */
  //删除所有key==null的entry
    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);
        }
    }
}

}