3.ThreadLocal源码

ThreadLocal

属于java.lang,提供线程局部变量,用来处理多线程并发的。

使用ThreadLocal维护变量时,ThreadLocal为每个使用该变量的线程提供独立的变量副本。
因此每一个线程都可以独立地改变自己的副本,而不会影响其它线程所对应的副本。

机制:

每个线程有一个自己的 ThreadLocalMap 对象
每一个 ThreadLocal 对象有一个创建时生成唯一的 Id
访问一个 ThreadLocal 变量的值,就是用这个唯一 Id 去 本线程 的 ThreadLocalMap 中查找对应的值

**简单具体点说,ThreadLocal类中有一个Map,用于存储每一个线程的变量副本,Map中元素的键为线程对象,而值对应线程的变量副本,由于Key值不可重复,每一个“线程对象”对应线程的“变量副本”,而到达了线程安全。
**

源码解析:

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

public class ThreadLocal {

private final int threadLocalHashCode = nextHashCode();

//从0开始,原子更新下一个,
private static AtomicInteger nextHashCode =
    new AtomicInteger();


//hash code 从 0 开始不断累加 0x61c88647 生成的。0x61c88647,目的为了让 hash code 能更好地分布在尺寸为 2 的 N 次方的数组里
private static final int HASH_INCREMENT = 0x61c88647;

//返回下个HashCode
private static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT);
}

//返回当前线程初始值作为当前ThreadLocal变量,返回null
protected T initialValue() {
    return null;
}


public ThreadLocal() {
}


//返回ThreadLocal变量在当前线程副本的值。如果该变量在当前线程中没有值,返回初始化的值。T泛型
public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);//得到map的实体
        if (e != null)
            return (T)e.value;
    }
    return setInitialValue();
}

//设置成初始值,并返回。
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;
}

//设置成特定的值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);
}
//移除当前线程中的threadLocal
 public void remove() {
     ThreadLocalMap m = getMap(Thread.currentThread());
     if (m != null)
         m.remove(this);
 }

//获取线程t的ThreadLocalMap;
//在Thread类中有属性,也就是线程附属的一个ThreadLocalMap threadLocals: 
//ThreadLocal.ThreadLocalMap threadLocals = null;
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals; 
}

//创建线程的threadLocals
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

//静态方法,创建继承于父线程的ThreadLocalMap
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    return new ThreadLocalMap(parentMap);
}

//一种嵌入式的替代吧,在子类中已经重新定义。在这里定义是为了提供createInheritedMap的工厂方法而不需在InheritableThreadLocal再子类化map class。
T childValue(T parentValue) {
    throw new UnsupportedOperationException();
}

 
//内部类ThreadLocalMap,哈希map来维护线程局部变量。没有操作暴露在ThreadLocal类之外。
//访问权限为package private 的内部类,因此可以在 Thread 类中引用(都是 java.lang 包下的)
//哈希表实体用WeakReferences作为key,来维持大而长时间的使用。
static class ThreadLocalMap {

    //泛型T 是 ThreadLocal
    static class Entry extends WeakReference {
        /** The value associated with this ThreadLocal. */
        Object value;

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

       //初始容量,必须是2的幂
    private static final int INITIAL_CAPACITY = 16;
    
    private Entry[] table;
    
    private int size = 0;

   //要调整的阈
    private int threshold; // Default to 0

   //设置阈 最少 len * 2 / 3
    private void setThreshold(int len) {
        threshold = len * 2 / 3;
    }
    
     // i自增 (模 len):加1操作,下一个索引
    private static int nextIndex(int i, int len) {
        return ((i + 1 < len) ? i + 1 : 0);
    }

   //i自减 (模 len):减1操作,上一个索引
    private static int prevIndex(int i, int len) {
        return ((i - 1 >= 0) ? i - 1 : len - 1);
    }

    //构造包含(firstKey, firstValue)的Map,懒惰构造,仅当至少有entry扔进的时候创建。
    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);
    }

    //带参parentMap构造函数(与父线程有关),只允许createInheritedMap调用。
    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 key = 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++;
                }
            }
        }
    }

    //返回该key关联的Entry。直接命中或者getEntryAfterMiss()
    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);
    }

    //未直接命中,便使用该方法找Entry。
    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;
    }

       //设置该key关联的value。
    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();
    }

   //移除该key关联的Entry
    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;
            }
        }
    }

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

         
        //往回找stale entry
        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;

                // 如果前面存在stale entry ,那么从它开始删。
                if (slotToExpunge == staleSlot)
                    slotToExpunge = i;
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);//clean
                return;
            }

            if (k == null && slotToExpunge == staleSlot)
                slotToExpunge = i;
        }

        // key没找到, 放new entry 在stale slot
        tab[staleSlot].value = null;
        tab[staleSlot] = new Entry(key, value);

        if (slotToExpunge != staleSlot)
            cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);//clean
    }

    //通过重散列在staleSlot和下一个为空的slot之间的所有可能冲突的项目,删除stale entry。
    //这个操作其实删除了其他在后一个null之前的stale entries.
    //返回下一个为空的index
    private int expungeStaleEntry(int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;

        // 清除staleSlot位置的entry,也就是置为null,size减1
        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);//重散列
                if (h != i) {
                    tab[i] = null;

                    while (tab[h] != null)
                        h = nextIndex(h, len);
                    tab[h] = e;
                }
            }
        }
        return i;
    }

    //remove,从i下一个开始,n是scan控制,log2(n)个cells被查询, 有stale entry情况。
    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) {
                n = len;
                removed = true;
                i = expungeStaleEntry(i);
            }
        } while ( (n >>>= 1) != 0);   //无符号右移一位
        return removed;  // 有remove的就会返回true
    }

     
    //显示移除stale entries,如果size >= threshold - threshold / 4,则resize()
    private void rehash() {
        expungeStaleEntries();

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

    //二倍扩容。
    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;
    }

    //删除过期的entries:e.get() == null
    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);
        }
    }
}
}

说明:
1.0x61c88647:数学中的神秘数字,神奇至极啊

This number represents the golden ratio (sqrt(5)-1) times two to the power of 31. The result is then a golden number, either 2654435769 or -1640531527.

2.示例1:私有静态 ThreadLocal
实例(serialNum
)为调用该类的静态 SerialNum.get()
方法的每个线程维护了一个“序列号”,该方法将返回当前线程的序列号。(线程的序列号是在第一次调用 SerialNum.get()
时分配的,并在后续调用中不会更改。)

public class ThreadLocalTest1 {

private static int nextSerialNum = 0; //静态变量

private static ThreadLocal serialNum = new ThreadLocal() {
    protected synchronized Object initialValue() {   // 同步方法
        return new Integer(nextSerialNum++); 
    }
};

public static int get() {
    return ((Integer) (serialNum.get())).intValue();
}}

3.示例2:在线程类内部创建ThreadLocal :线程 隔离

(1)、在多线程的类(如ThreadDemo类)中,创建一个ThreadLocal对象threadXxx,用来保存线程间需要隔离处理的对象xxx。

(2)、在ThreadDemo类中,创建一个获取要隔离访问的数据的方法getXxx(),在方法中判断,若ThreadLocal对象为null时候,应该new()一个隔离访问类型的对象,并强制转换为要应用的类型。

(3)、在ThreadDemo类的run()方法中,通过调用getXxx()方法获取要操作的数据,这样可以保证每个线程对应一个数据对象,在任何时刻都操作的是这个对象。

代码参考下面的第二个网址。

4 示例 线程安全

public class ConnectionManager {  

private static ThreadLocal connectionHolder = new ThreadLocal() {  
    @Override  
    protected Connection initialValue() {  
        Connection conn = null;  
        try {  
            conn = DriverManager.getConnection(  
                    "jdbc:mysql://localhost:3306/test", "username",  
                    "password");  
        } catch (SQLException e) {  
            e.printStackTrace();  
        }  
        return conn;  
    }  
};  

public static Connection getConnection() {  
    return connectionHolder.get();  
}  

public static void setConnection(Connection conn) {  
    connectionHolder.set(conn);  
}  }  

参考:

http://www.tuicool.com/articles/2QjEzm
http://blog.csdn.net/vking_wang/article/details/14225379

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