ThreadLocal原理和使用场景

简介

ThreadLocal是一个关于创建线程局部变量的类。

通常情况下，我们创建的成员变量都是线程不安全的。因为他可能被多个线程同时修改，此变量对于多个线程之间彼此并不独立，是共享变量。而使用ThreadLocal创建的变量只能被当前线程访问，其他线程无法访问和修改。也就是说：将线程公有化变成线程私有化（空间换时间）。

核心源码分析

注意：ThreadLocal通常都定义为static，ThreadLocal没有存储功能，变量副本的真实存储位置是Thread对象的threadLocals这个ThreadLocal.ThreadLocalMap变量中，可以将ThreadLocal理解为一个工具类，用来保证线程本地变量的存储和存储碰撞。

    public static void main(String[] args){
        ThreadLocal threadLocal = new ThreadLocal();
        threadLocal.set("haha");
        System.out.println(threadLocal.get());
    }

ThreadLocal(为key)

set

创建一个和当前线程强相关的容器，把值放到这个容器里，线程之间容器相互隔离。

    /**
     * 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);
    }

    /**
     * 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);
    }

此段代码可以看出，我们在set的时候，是获取了一个ThreadLocalMap对象（上述当中的容器），它是当前线程的一个参数，如果已经建立了，以threadlocal为key,直接往里面塞值，没有建立的话需要初始化一下。

get

    /**
     * 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;
    }

获取线程中ThreadLocalMap已经存储的内容，如果ThreadLocalMap还没创建，则创建map并赋null。

remove

    /**
     * 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);
     }

        /**
         * 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;
                }
            }
        }

remove就是将存储的变量清空。

ThreadlocalMap

set

        /**
         * 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;
            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();
        }

先用hash码与长度取与，然后找到对应数组位置index,如果没有发生碰撞，就按指定位置存储，如果发生碰撞，就顺序找个空的位置存储。

get

        /**
         * 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);
        }
 
        /**
         * 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);
                else
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }

查找和放入是一样的道理。

remove

        /**
         * 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;
                }
            }
        }

看完代码，我们总结一下ThreadLocal它源码的核心几个方法及其作用

• set()方法用于保存当前线程的副本变量值。
• get()方法用于获取当前线程的副本变量值。
• initialValue()为当前线程初始副本变量值。
• remove()方法移除当前线程的副本变量值。

抽象图示

ThreadLocal弱引用

To help deal with very large and long-lived usages, the hash table entries use WeakReferences for keys.
为了处理非常大和生命周期非常长的线程，哈希表使用弱引用作为 key。

通常ThreadLocalMap的生命周期跟Thread（注意线程池中的Thread）一样长，如果没有手动删除对应key（线程使用结束归还给线程池了，其中的KV不再被使用但又不会GC回收，可认为是内存泄漏），一定会导致内存泄漏，但是使用弱引用可以多一层保障：弱引用ThreadLocal会被GC回收，不会内存泄漏，对应的value在下一次ThreadLocalMap调用set,get,remove的时候会被清除，Java8已经做了上面的代码优化。

使用场景

1.线程安全的时间工具类

package com.test;
 
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
 
public class DateUtil {
 
    private static final String date_format = "yyyy-MM-dd HH:mm:ss";
 
    private static ThreadLocal threadLocal = new ThreadLocal();
 
    public static DateFormat getDateFormat(String dateFormate) {
        DateFormat df = threadLocal.get();
        if(df == null){
            df = new SimpleDateFormat(dateFormate);
            threadLocal.set(df);
        }
        return df;
    }
 
    public static String formatDate(Date date,String dateFormate) throws ParseException {
        return getDateFormat(dateFormate).format(date);
    }
 
    public static Date parse(String strDate,String dateFormate) throws ParseException {
        return getDateFormat(dateFormate).parse(strDate);
    }
 
}

2.B端存储用户信息

public class UserThreadLocal {
    private  UserThreadLocal(){}
 
    private static final ThreadLocal LOCAL = new ThreadLocal<>();
 
    public static void put(SysUser sysUser){
        LOCAL.set(sysUser);
    }
    public static SysUser get(){
        return LOCAL.get();
    }
 
    public static void remove(){
        LOCAL.remove();
    }
}