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Java中ThreadLocal导致内存OOM的原因是什么

java ThreadLocal oom
128 2023-04-28

Java中ThreadLocal导致内存OOM的原因是什么

原因分析

ThreadLocal 导致内存 OOM 的原因是什么?

ThreadLocal 底层通过 ThreadLocalMap 存储数据

源码如下: 

  • 当我们使用ThreadLocal.set()时,set的value与key(即业务自己定义的ThreadLocal类)会存储在ThreadLocalMap的Entry[]数组里

源码如下:

  • 其中Entry是实现了一个弱引用WeakReference,Entry的key(即业务方定义的 ThreadLocal类)会被包装成一个弱引用当成Entry的key。Java的弱引用的定义是,当JVM执行垃圾回收扫描的时候,当发现只有弱引用的对象时,会立即回收此对象,这是ThreadLocal当初设计的时候防止内存溢出的一个手段

源码如下: 

虽然key被包装成了一个弱引用会被垃圾回收机制给回收,但是value在线程(Thread)不死亡时却可能存在一条强引用链.

由于 value是强引用,只要 Thread不死亡时,例如线程池,这条强引用链就会存在,那么value就不会回收,可能造成内存溢出

引用关系如下: Thread ==> ThreadLocalMap ==> Entry ==> value

但是这个消除强引用链的动作是需要业务方在get的情况下触发的,可能业务方并不会get、也可能get是key不为空,并不会触发 expungeStaleEntry 类。所以开发者要养成良好的习惯,记得用完 ThreadLocal 时,调一次ThreadLocal.remove()方法或者 ThreadLocal.set(null)

正确的使用方式

public class ThreadLocalTest {

    /**

     * 未初始化的本地线程变量

     */

    private static ThreadLocal<User> userThreadLocal = new ThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {

        int threadNum = 10;

        List<Thread> threadList = Lists.newArrayList();



        for (int i = 0; i < threadNum; ++i) {

            long userId = i;

            Thread t = new Thread(() -> {

                try {

                    // 设置变量值

                    userThreadLocal.set(new User(userId, "lanxing" + userId, "2x"));

                    // 使用变量

                    doSomething();

                } finally {

                    // 移除变量

                    userThreadLocal.remove();   //移除ThreadLocal变量

                }

            }, "T" + i);

            threadList.add(t);

            t.start();

        }



        for (int i = 0; i < threadNum; ++i) {

            threadList.get(i).join();

        }

    }

    private static void doSomething() {

        log.info("Use ThreadLocal variable :{}", JSON.toJSONString(userThreadLocal.get()));

    }

}

@Data

@NoArgsConstructor

@AllArgsConstructor

class User {

    private Long id;

    private String name;

    private String level;

}

打印结果:

14:30:26.790 [T2] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":2,"level":"2x","name":"lanxing2"}

14:30:26.789 [T5] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":5,"level":"2x","name":"lanxing5"}

14:30:26.792 [T0] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":0,"level":"2x","name":"lanxing0"}

14:30:26.792 [T4] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":4,"level":"2x","name":"lanxing4"}

14:30:26.792 [T8] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":8,"level":"2x","name":"lanxing8"}

14:30:26.791 [T1] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":1,"level":"2x","name":"lanxing1"}

14:30:26.792 [T7] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":7,"level":"2x","name":"lanxing7"}

14:30:26.792 [T6] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":6,"level":"2x","name":"lanxing6"}

14:30:26.791 [T9] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":9,"level":"2x","name":"lanxing9"}

14:30:26.790 [T3] INFO io.zhengsh.order.tool.ThreadLocalTest - Use ThreadLocal variable :{"id":3,"level":"2x","name":"lanxing3"}