使用ThreadLocal要注意

ThreadLocalMap使用ThreadLocal的弱引用作为key，如果一个ThreadLocal没有外部强引用来引用它，那么系统 GC 的时候，这个ThreadLocal势必会被回收，这样一来，ThreadLocalMap中就会出现key为null的Entry，就没有办法访问这些key为null的Entry的value，如果当前线程再迟迟不结束的话，这些key为null的Entry的value就会一直存在一条强引用链：Thread Ref -> Thread -> ThreaLocalMap -> Entry -> value永远无法回收，造成内存泄漏。

我们编写一个ThreadLocalMap正确使用的示例：

//ThreadLocal应用实例
public class ThreadLocalApp {
  
     public static final ThreadLocal threadLocal = new ThreadLocal();
  
     public static void muti2() {
         int i[] = (int[]) threadLocal.get();
         i[1] = i[0] * 2;
         threadLocal.set(i);
     }
  
     public static void muti3() {
         int i[] = (int[]) threadLocal.get();
         i[2] = i[1] * 3;
         threadLocal.set(i);
     }
  
     public static void muti5() {
         int i[] = (int[]) threadLocal.get();
         i[3] = i[2] * 5;
         threadLocal.set(i);
     }
  
     public static void main(String args[]) {
         for (int i = 0; i < 5; i++) {
             new Thread() {
                 public void run() {
                     int start = new Random().nextInt(10);
                     int end[] = {0, 0, 0, 0};
                     end[0] = start;
                     threadLocal.set(end);
                     ThreadLocalApp.muti2();
                     ThreadLocalApp.muti3();
                     ThreadLocalApp.muti5();
                     //int end = (int) threadLocal.get();
                     System.out.println(end[0] + "  " + end[1] + "  " + end[2] + "  " + end[3]);
                     threadLocal.remove();
                 }
             }.start();
         }
     }
 }