ThreadLocal源码阅读

如何使用

ThreadLocal 的使用比较简单,日常使用基本也只涉及存值和取值。

public class ThreadLocalTest {
    public static  ThreadLocal<String> threadLocal  = new ThreadLocal<>();
    @Test
    public void userThreadLocal(){
        threadLocal.set("123");
        String str = threadLocal.get();
        System.out.println(str);
    }
}

如何实现

整体设计思路如下：

• ThreadLocal中存在一个内部类ThreadLocalMap，用于存储需要存放在线程局部变量的数据，其key为当前ThreadLocal，其value为我们存入的值
• Thread类持有ThreadLocal.ThreadLocalMap变量
• 当前线程通过ThreadLocal的set方法在当前线程内部的ThreadLocal.ThreadLocalMap变量中以当前的threadLocal为key存入值；通过get方法，以当前的threadLocal为key从当前线程内部的ThreadLocalMap中取出存入的值
• 这样设计的效果就是每个线程存入值后都可以通过同一个ThreadLocal.get回各自存入的内容

关键方法

• set(T value)方法

//ThreadLcoal.set()
public void set(T value) {
        Thread t = Thread.currentThread();//1.获得当前线程对象
        ThreadLocalMap map = getMap(t);//2.获取当前线程内部的ThreadLocal.ThreadLocalMap对象
        if (map != null)
            map.set(this, value);//3.map不为空时以当前threadLocal为key存入value
        else
            createMap(t, value);//4.map为空时创建map
    }

//2处的getMap方法，ThreadLcoal.getMap()
ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;//返回当前线程内部的ThreadLocal.ThreadLocalMap对象
    }

//4处的createMap方法，ThreadLcoal.createMap()
void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

• get()方法

//ThreadLcoal.get()
public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);//1.获取当前线程的ThreadLocal.ThreadLocalMap对象
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);//2.当前线程的ThreadLocal.ThreadLocalMap对象不为空则以当前threadLocal为key取出Map内部的实体
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;//3.e.value即是我们存入的值
                return result;
            }
        }
        return setInitialValue();//4.当map为空时初始化map
    }

//4处的setInitialValue方法，ThreadLcoal.setInitialValue()
  private T setInitialValue() {
        T value = initialValue();//初始值为null
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)//get中map为空才走的这个方法，且map仅属于当前线程，不存在线程安全问题，感觉没必要再次判空。欢迎指教
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

测试

public class ThreadLocalTest {
    public static  ThreadLocal<String> threadLocal  = new ThreadLocal<>();

    @Test
    public void testThreadLocal() throws InterruptedException {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1,5,1, TimeUnit.MINUTES,new LinkedBlockingDeque<>());
        //第一个线程操作threadLocal
        threadPoolExecutor.execute(()->{
            threadLocal.set("first thread");
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("firstThread.get:  "+threadLocal.get());
        });
        //第二个线程操作threadLocal
        threadPoolExecutor.execute(()->{
            threadLocal.set("second thread");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("secondThread.get:  "+threadLocal.get());
        });
        //主线程等待线程池内线程完成任务处理
        threadPoolExecutor.awaitTermination(1,TimeUnit.MINUTES);
    }
}

结果输出

补充：为什么ThreadLocalMap中Entry的key要使用软引用

static class ThreadLocalMap {

        static class Entry extends WeakReference<ThreadLocal> {
            Object value;
            Entry(ThreadLocal k, Object v) {//此处的key为软引用
                super(k);
                value = v;
            }
        }
        //省略其他代码。。。
 }

因为这个key是指向ThreadLocal的，请看如下代码：

public void testThreadLocal() {
        ThreadLocal threadLocal  = new ThreadLocal<String>(); 
         threadLocal.set("abc"); 
         threadLocal.get(); 
}

当此方法出栈时threadLocal本应被回收，但是若ThreadLocalMap.Entry的key为强引用的话threadLocal在当前线程消亡之前都不会被回收，存在内存泄漏的风险