ThreadLocal

ThreadLocal：是线程的本地变量，也叫本地存储。ThreadLocal为变量在每个线程中创建了一个副本，那么每个线程可以访问自己内容的副本变量

ThreadLocal应用场景：

　　比如

class ConnectionManager {
     
    private static Connection connect = null;
     
    public static Connection openConnection() {
        if(connect == null){
            connect = DriverManager.getConnection();
        }
        return connect;
    }
     
    public static void closeConnection() {
        if(connect!=null)
            connect.close();
    }
}

这是一个数据库连接管理类，当多个线程同时调用的时候就会出现线程安全问题。

避免线程安全问题：

　　可以在方法前加synchronized修饰，但是这样就会大大影响了效率

这里的这个例子可以不用connection共享，那么可能有的人就会想到，既然不用共享，那么我直接每个线程new一个出来。虽然这样是不会造成线程安全问题，而且线程与线程之间不必相互等待了，但是这样可能会导致服务器压力太大，因为每个线程都有一个connection，会严重影响性能

class ConnectionManager {
     
    private  Connection connect = null;
     
    public Connection openConnection() {
        if(connect == null){
            connect = DriverManager.getConnection();
        }
        return connect;
    }
     
    public void closeConnection() {
        if(connect!=null)
            connect.close();
    }
}
 
 
class Dao{
    public void insert() {
        ConnectionManager connectionManager = new ConnectionManager();
        Connection connection = connectionManager.openConnection();
         
        //使用connection进行操作
         
        connectionManager.closeConnection();
    }
}

那么出现这种情况我们可以用ThreadLocal类

ThreadLocal 提供了四个方法

public T get() { }
public void set(T value) { }
public void remove() { }
protected T initialValue() { }

get（）获取ThreadLocal在当前线程中保存的变量副本

set（）用来设置当前线程中变量副本

remove（）用来移除当前线程变量副本

initialValue（）是一个protected方法，一般用来重写，是一个延迟加载方法

 /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

get方法会掉用getMap()方法，如果map不为空，然后会取到<key,value>键值对

  /**
         * 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);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

注意这里取键值对传的是this，而不是当前线程t。

如果map为空则调用setInit方法返回value

 /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

getMap方法返回的是threadLocals，而threadlocals实际上就是一个ThreadLocalMap对象

 /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

ThreadLocalMap内部实现

static class ThreadLocalMap {

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

实际上是取的ThreadLocal作为键值

set方法

 /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    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;
    }

当map不为空，则设置键值，否则创建map

/**
     * Create the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the map
     * @param map the map to store.
     */
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

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

每个线程内部有一个ThreadLocalMap的成员变量threadLocals，这个变量实际上是来存储实际变量副本的，键值为当前ThreadLocal、变量，value为变量副本

初始时，threadLocals为空，会通过get（），set（）方法对threadLocals初始化，并且以当前线程变量为键值进行赋值

在当前线程里面，如果要使用本地变量，就可以通过get方法在threadLocals里面查找

public class Test {
    ThreadLocal<Long> longLocal = new ThreadLocal<Long>();
    ThreadLocal<String> stringLocal = new ThreadLocal<String>();
 
     
    public void set() {
        longLocal.set(Thread.currentThread().getId());
        stringLocal.set(Thread.currentThread().getName());
    }
     
    public long getLong() {
        return longLocal.get();
    }
     
    public String getString() {
        return stringLocal.get();
    }
     
    public static void main(String[] args) throws InterruptedException {
        final Test test = new Test();
         
         
        test.set();
        System.out.println(test.getLong());
        System.out.println(test.getString());
     
         
        Thread thread1 = new Thread(){
            public void run() {
                test.set();
                System.out.println(test.getLong());
                System.out.println(test.getString());
            };
        };
        thread1.start();
        thread1.join();
         
        System.out.println(test.getLong());
        System.out.println(test.getString());
    }
}

输出结果

1

main

2

thread0

1

main

所以可以看出，threadLocal存的是线程的本地变量，main线程，和thread1线程存取的值都不同