JavaThreadLocal使用和源码分析

ThreadLocal使用和源码分析

概述

• ThreadLocal即线程局部变量。
• ThreadLocal提供线程的局部变量&＃xff0c;每个线程可以通过了get/set方法对局部变量进行操作。
• 局部变量不会和其他线程的局部变量有冲突&＃xff0c;实现了线程的数据隔离。
• Thread和ThreadLocal关系&＃xff1a;
  • Thread内部持有ThreadLocalMap类型的成员变量threadLocals。
  • ThreadLocal是负责操作线程局部变量的管理类&＃xff0c;有get()、set()、remove()方法负责获取、设置、清除数据。
  • ThreadLocalMap是ThreadLocal的内部类&＃xff0c;用于存储线程的局部变量&＃xff0c;内部维护一个Entry数组&＃xff0c;键值对结构&＃xff0c;键为ThreadLocal类型&＃xff0c;值为Object类型。

基本使用

//创建ThreadLocal对象
private static ThreadLocal<String> mThreadLocal &＃61; new ThreadLocal<>();static class A implements Runnable {&＃64;Overridepublic void run() {//设置数据mThreadLocal.set("线程A");//覆盖数据mThreadLocal.set("线程AAA");//获取数据System.out.println(mThreadLocal.get());}
}static class B implements Runnable {&＃64;Overridepublic void run() {mThreadLocal.set("线程B");System.out.println(mThreadLocal.get());}
}public static void main(String[] args) {mThreadLocal.set("线程main");new Thread(new A()).start();new Thread(new B()).start();System.out.println(mThreadLocal.get());
}


输出信息&＃xff1a;

线程main
线程AAA
线程B


源码分析

Thread

public class Thread implements Runnable {ThreadLocal.ThreadLocalMap threadLocals &＃61; null;private void exit() { threadLocals &＃61; null;}
}


ThreadLocal

public class ThreadLocal<T> {//创建ThreadLocalMap对象void createMap(Thread t, T firstValue) {t.threadLocals &＃61; new ThreadLocalMap(this, firstValue);}//获取ThreadLocalMap对象ThreadLocalMap getMap(Thread t) {return t.threadLocals;}//设置ThreadLocal变量的值//获取当前线程&＃xff0c;再获取ThreadLocalMap对象&＃xff0c;如果map不为null进行set操作&＃xff0c;map为null则进行创建操作public void set(T value) {Thread t &＃61; Thread.currentThread();ThreadLocalMap map &＃61; getMap(t);if (map !&＃61; null)map.set(this, value);elsecreateMap(t, value);}//获取ThreadLocal变量的值public T get() {Thread t &＃61; Thread.currentThread();ThreadLocalMap map &＃61; getMap(t);if (map !&＃61; null) {ThreadLocalMap.Entry e &＃61; map.getEntry(this);if (e !&＃61; null) {T result &＃61; (T)e.value;return result;}}return setInitialValue();}//初始化ThreadLocal的值private T setInitialValue() {T value &＃61; initialValue();Thread t &＃61; Thread.currentThread();ThreadLocalMap map &＃61; getMap(t);if (map !&＃61; null)map.set(this, value);else//创建createMap(t, value);return value;}//清除数据public void remove() {ThreadLocalMap m &＃61; getMap(Thread.currentThread());if (m !&＃61; null)m.remove(this);}private void remove(ThreadLocal<?> key) {Entry[] tab &＃61; table;int len &＃61; tab.length;int i &＃61; key.threadLocalHashCode & (len-1);for (Entry e &＃61; tab[i];e !&＃61; null;e &＃61; tab[i &＃61; nextIndex(i, len)]) {if (e.get() &＃61;&＃61; key) {e.clear();expungeStaleEntry(i);return;}}}
}


ThreadLocalMap

static class ThreadLocalMap {//存储数据static class Entry extends WeakReference<ThreadLocal<?>> {Object value;Entry(ThreadLocal<?> k, Object v) {super(k);value &＃61; v;}}//table初始容量private static final int INITIAL_CAPACITY &＃61; 16;//table用于存储数据private Entry[] table;//下次扩容的阀值private int threshold; //2/3的负载因子private void setThreshold(int len) {threshold &＃61; len * 2 / 3;}//初始化ThreadLocalMap&＃xff0c;key为ThreadLocalThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {table &＃61; new Entry[INITIAL_CAPACITY];int i &＃61; firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);table[i] &＃61; new Entry(firstKey, firstValue);size &＃61; 1;setThreshold(INITIAL_CAPACITY);}//设置数据private void set(ThreadLocal<?> key, Object value) { Entry[] tab &＃61; table;int len &＃61; tab.length;//通过hash值获取索引值int i &＃61; key.threadLocalHashCode & (len-1);//遍历tab如果存在则更新for (Entry e &＃61; tab[i];e !&＃61; null;e &＃61; tab[i &＃61; nextIndex(i, len)]) {ThreadLocal<?> k &＃61; e.get();//key相同&＃xff0c;覆盖值if (k &＃61;&＃61; key) {e.value &＃61; value;return;}//key为null&＃xff0c;清空所有key为null的数据if (k &＃61;&＃61; null) {replaceStaleEntry(key, value, i);return;}}//若不存在则直接添加tab[i] &＃61; new Entry(key, value);int sz &＃61; &＃43;&＃43;size;if (!cleanSomeSlots(i, sz) && sz >&＃61; threshold)rehash();}//获取数据private Entry getEntry(ThreadLocal<?> key) {int i &＃61; key.threadLocalHashCode & (table.length - 1);Entry e &＃61; table[i];if (e !&＃61; null && e.get() &＃61;&＃61; key)return e;elsereturn getEntryAfterMiss(key, i, e);}
}


内存泄漏问题

ThreadLocalMap内部维护了一个Entry数组&＃xff0c;在没有执行remove的情况下&＃xff0c;将ThreadLocal设置为null&＃xff0c;下次GC时&＃xff0c;key是弱引用会被回收&＃xff0c;value是强引用并指向对象就会一直存在内存中&＃xff0c;这样会发生内存泄露。

所以ThreadLocal使用后需要及时remove掉。