ThreadLocal总结
- ThreadLocal作用：为每一个线程开辟一个副本，意思是线程本地
1、代码证明ThreadLocal为每个线程创建线程
线程==>
public class ThreadDemo extends Thread {
ThreadLocal<Long> threadLocal;
public ThreadDemo(ThreadLocal<Long> threadLocal) {
this.threadLocal = threadLocal;
}
@Override
public void run() {
Long result = threadLocal.get();
if (result == null) {
threadLocal.set(System.currentTimeMillis());
System.out.println(Thread.currentThread().getName() + "===>" + threadLocal.get()+"===>"+threadLocal);
}
}
}
证明==>
public class ThreadLocalDemo {
public static ThreadLocal<Long> threadLocal = new ThreadLocal();
public static void main(String[] args) {
ThreadDemo t1 = new ThreadDemo(threadLocal);
ThreadDemo t2 = new ThreadDemo(threadLocal);
t1.start();
try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); }
t2.start();
}
}
2、为什么为线程创建副本：分析ThreadLocal的get/set方法
set=====>
public void set(T value) {
//获取当前线程
Thread t = Thread.currentThread();
//获得当前线程的成员变量，类型为ThreadLocal.ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null)
//key:this，就是当前线程；value：就是存入的泛型值
map.set(this, value);
else
createMap(t, value);
}
get ====>
public T get() {
//获取当前线程
Thread t = Thread.currentThread();
//获得当前线程的成员变量，类型为ThreadLocal.ThreadLocalMap
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();
}
3、实际解决了什么？
mybatis内部sqlsession其内部就是采用ThreadLocal
爬虫每个Request的代理利用ThreadLocal
4、产生了什么问题？
内存泄露；
比如线程池核心线程之外的线程执行完任务，但是ThreadLocal没有释放，就会导致内存泄露
解决：可以通过delete()删除
