ThreadLocal

1.介绍

      深挖过threadLocal之后，一句话概括：Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。所以ThreadLocal的应用场合，最适合的是按线程多实例（每个线程对应一个实例）的对象的访问，并且这个对象很多地方都要用到。

      早在JDK 1.2的版本中就提供java.lang.ThreadLocal，ThreadLocal为解决多线程程序的并发问题提供了一种新的思路。使用这个工具类可以很简洁地编写出优美的多线程程序。当使用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，所以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。

　　从线程的角度看，目标变量就象是线程的本地变量，这也是类名中“Local”所要表达的意思。所以，在Java中编写线程局部变量的代码相对来说要笨拙一些，因此造成线程局部变量没有在Java开发者中得到很好的普及。

ThreadLocal、ThreadLocal、Thread之间的关系

　　ThreadLocalMap是ThreadLocal内部类，由ThreadLocal创建，用来存储数据，采用类似hashmap机制，存储了以threadLocal为key，需要隔离的数据为value的Entry键值对数组结构。Thread有ThreadLocal.ThreadLocalMap类型的属性。

        ThreadLocal，有个ThreadLocalMap类型的属性，连接ThreadLocalMap和Thread。来处理Thread的TheadLocalMap属性，包括init初始化属性赋值、get对应的变量，set设置变量等。通过当前线程，获取线程上的ThreadLocalMap属性，对数据进行get、set等操作。

源码如下：

Thread的属性：

public
class Thread implements Runnable {
    /*...其他属性...*/
 
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
 
    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

ThreadLocal和ThreadLocalMap:

public class ThreadLocal<T> {
    /**..其他属性和方法稍后介绍...*/
    /**
     * ThreadLocalMap is a customized hash map suitable only for
     * maintaining thread local values. No operations are exported
     * outside of the ThreadLocal class. The class is package private to
     * allow declaration of fields in class Thread.  To help deal with
     * very large and long-lived usages, the hash table entries use
     * WeakReferences for keys. However, since reference queues are not
     * used, stale entries are guaranteed to be removed only when
     * the table starts running out of space.
     */
    static class ThreadLocalMap {

由ThreadLocal对Thread的TreadLocalMap进行赋值:

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

2.使用

ThreadLocal类核心方法set、get、initialValue、withInitial、setInitialValue、remove：

   /**
     * Returns the current thread's "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the {@link #get}
     * method, unless the thread previously invoked the {@link #set}
     * method, in which case the {@code initialValue} method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of {@link #remove} followed by {@link #get}.
     *
     * <p>This implementation simply returns {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }
 
    /**
     * Creates a thread local variable. The initial value of the variable is
     * determined by invoking the {@code get} method on the {@code Supplier}.
     *
     * @param <S> the type of the thread local's value
     * @param supplier the supplier to be used to determine the initial value
     * @return a new thread local variable
     * @throws NullPointerException if the specified supplier is null
     * @since 1.8
     */
    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }
 
    /**
     * Creates a thread local variable.
     * @see #withInitial(java.util.function.Supplier)
     */
    public ThreadLocal() {
    }
 
    /**
     * 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) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }
 
    /**
     * 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;
    }
 
    /**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
 
    /**
     * Removes the current thread's value for this thread-local
     * variable.  If this thread-local variable is subsequently
     * {@linkplain #get read} by the current thread, its value will be
     * reinitialized by invoking its {@link #initialValue} method,
     * unless its value is {@linkplain #set set} by the current thread
     * in the interim.  This may result in multiple invocations of the
     * {@code initialValue} method in the current thread.
     *
     * @since 1.5
     */
     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

• initialValue返回该线程局部变量的初始值。该方法是一个protected的方法，显然是为了让子类覆盖而设计的。这个方法是一个延迟调用方法，在线程第1次调用get()或set(Object)时才执行，并且仅执行1次。ThreadLocal中的缺省实现直接返回一个null。
• withInitial提供一个Supplier的lamda表达式用来当做初始值，java8引入。
• setInitialValue设置初始值。在get操作没有对应的值时，调用此方法。private方法，防止被覆盖。过程和set类似，只不过是用initialValue作为value进行设置。
• set设置当前线程对应的线程局部变量的值。先取出当前线程对应的threadLocalMap，如果不存在则用创建一个，否则将value放入以this，即threadLocal为key的映射的map中，其实threadLocalMap内部和hashMap机制一样，存储了Entry键值对数组，后续会深挖threadLocalMap。
• get该方法返回当前线程所对应的线程局部变量。和set类似，也是先取出当前线程对应的threadLocalMap，如果不存在则用创建一个，但是是用inittialValue作为value放入到map中，且返回initialValue，否则就直接从map取出this即threadLocal对应的value返回。
• remove将当前线程局部变量的值删除，目的是为了减少内存的占用，该方法是JDK 5.0新增的方法。需要指出的是，当线程结束后，对应该线程的局部变量将自动被垃圾回收，所以显式调用该方法清除线程的局部变量并不是必须的操作，但它可以加快内存回收的速度。需要注意的是，如果remove之后又调用了get，会重新初始化一次，即再次调用initialValue方法，除非在get之前调用set设置过值。

3.案例

本案例在多线程中对ThreadLocal和线程局部变量进行测试

1.首先，测试一下线程全局变量在多个线程下的使用，代码如下：

import java.util.Random;

public class testThreadLocal extends Thread{ 6     
    private static boolean flag;
    
    public void run()
    {
        Random random=new Random();
        int a=random.nextInt(20);
                flag=flag(a);
        System.out.println("其他线程"+a+"启动，flag="+flag);
        }
        
    public static void main(String[] args) {    
        flag=flag(3);
        System.out.println("主线程启动，flag="+flag);
    
        for (int i = 0; i < 10; i++) {
            new testThreadLocal().start();
        }    
    }
    
    public static boolean flag(int a){
        if(a>4){
            flag=true;
            return flag;
        }else {
            return flag;
        }
    }
}
        

结果如下：

明显有问题，所以在多线程环境下，是不能使用线程全局变量的。

2.线程局部变量测试，代码如下：

import java.util.Random;

public class testThreadLocal extends Thread{
        public void run()
    {
        Random random=new Random();
        int a=random.nextInt(20);
        boolean flag=flag(a,false);
        System.out.println("其他线程"+a+"启动，flag="+flag);
    }
        public static void main(String[] args) {
        
        boolean flag=flag(3,false);
        System.out.println("主线程启动，flag="+flag);
    
        for (int i = 0; i < 10; i++) {
            new testThreadLocal().start();
        }
        
    }
    
    public static boolean flag(int a，boolean flag){
        if(a>4){
            flag=true;
            return flag;
        }else {
            return flag;
        }
    }
}

运行结果如下：

结果正常！

3.ThreadLocal测试

import java.util.Random;

public class testThreadLocal extends Thread{
    
    private static ThreadLocal<Boolean> tlflag=new ThreadLocal<>();

    public void run()
    {
        Random random=new Random();
        int a=random.nextInt(20);
        flag(a);
        System.out.println("ThreadLocal形式，其他线程"+a+"启动，flag="+tlflag.get());
    }
    
    public static void main(String[] args) {
        
        flag(3);
        System.out.println("ThreadLocal形式，主线程启动，flag="+tlflag.get());
        
        for (int i = 0; i < 10; i++) {
            new testThreadLocal().start();
        }
    }

        public static void flag(int a){
        
        if(a>4){
            tlflag.set(true);
        }else {
            tlflag.set(false);
        }
    }
}

运行结果为：

结果正常！

部分内容参考自博客：https://blog.csdn.net/lufeng20/article/details/24314381