深入学习ThreadLocal

一时失言乱红尘 2022-08-28 13:46 170阅读 0赞

今天深入学习了一下ThreadLocal，这里做一个记录。

目录：  
一。ThreadLocal简介  
二。ThreadLocal的简单使用  
三。ThreadLocal的实现原理  
3.1ThreadLocal的set方法  
3.2ThreadLocal的get方法  
3.3 ThreadLocal的setInitialValue()方法  
3.4ThreadLocal的remove()方法  
四。ThreadLocal的实际应用  
五。ThreadLocal不支持继承性  
六。InheritableThreadLocal类  
七。ThreadLocal和同步方法解决线程安全问题的区别

**一。ThreadLocal简介**

我们都知道，多线程访问共享变量的时候容易发生线程不安全的情况，为了解决线程不安全的问题，我们可以通过同步机制避免非线程安全。还有一种方式是使用ThreadLocal,也就是可以在不同的线程中维护一份共享变量的副本。这样可以在某些场景下使用可以避免非线程安全。
     ThreadLocal是JDK包提供的，它提供线程本地变量，如果创建了一个ThreadLocal变量，那么访问这个变量的每一个线程都会有这个变量的一个副本，在实际多线程操作的时候，操作的是自己本地内存中的变量，从而规避了线程安全问题。

**二。ThreadLocal的简单使用**

public class ThreadLocalDemo { 
    
        private static String currentStr = null;
    
        private static void init(){ 
            currentStr = "init";
        }
    
        public static void main(String[] args) { 
            System.out.println("main start");
            init();
    
            Thread thread1 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread1.start");
                    System.out.println("thread1-1:"+currentStr);
                    currentStr = "thread1-temp-end";
                    System.out.println("thread1-2::"+currentStr);
                    System.out.println("thread1.end");
                }
            });
    
            Thread thread2 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread2.start");
                    System.out.println("thread2-1:"+currentStr);
                    currentStr = "thread2-temp-end";
                    System.out.println("thread2-2:"+currentStr);
                    System.out.println("thread2.end");
                }
            });
    
            thread1.start();
            thread2.start();
    
            System.out.println("main start");
        }
    
    }

代码运行结果如下：

main start
    main start
    thread1.start
    thread1-1:init
    thread2.start
    thread2-1:init
    thread2-2:thread2-temp-end
    thread1-2::thread2-temp-end
    thread1.end
    thread2.end

由上看到

thread2-2:thread2-temp-end  
thread1-2::thread2-temp-end  
这两行就可以看出这是一个典型的存在线程安全问题的代码，两个线程操作了同一个变量，有可能会得到与臆想不一致的情况。

下面给出一个相同场景下使用了ThreadLocal的例子：

public class ThreadLocalDemo2 { 
    
        private static ThreadLocal<String> currentStr = new ThreadLocal<>();
    
        public static void main(String[] args) { 
            System.out.println("main start");
    
            Thread thread1 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread1.start");
                    System.out.println("thread1:currentStr1-1:"+currentStr.get());
                    currentStr.set("thread1-temp-end");
                    System.out.println("thread1:currentStr1-2:"+currentStr.get());
                    System.out.println("thread1.end");
                }
            });
    
            Thread thread2 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread2.start");
                    System.out.println("thread2:currentStr2-1:"+currentStr.get());
                    currentStr.set("thread2-temp-end");
                    System.out.println("thread2:currentStr2-2:"+currentStr.get());
                    System.out.println("thread2.end");
                }
            });
    
            thread1.start();
            thread2.start();
    
            System.out.println("main start");
        }
    
    }

他的运行结果如下：

main start
    thread1.start
    thread1:currentStr1-1:null
    main start
    thread2.start
    thread2:currentStr2-1:null
    thread2:currentStr2-2:thread2-temp-end
    thread2.end
    thread1:currentStr1-2:thread1-temp-end
    thread1.end

我们可以看到两个线程虽然共享了同一个currentStr对象，但是在自己线程中对currentStr变量的修改不会对另一个线程的currentStr对象可见，显然这样就避免了多线程共享数据线程不安全的问题。同时也能看出其实两个线程操作的是自己线程对currentStr变量的一个副本，所以不会影响别的线程的这个currentStr变量。

**三。ThreadLocal的实现原理**

首先我们来看一下Thread 类，会发现Thread类中有两个变量：

/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */
        ThreadLocal.ThreadLocalMap threadLocals = null;
    
        /* * InheritableThreadLocal values pertaining to this thread. This map is * maintained by the InheritableThreadLocal class. */
        ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

由上可以看到在Thread中维护了两个 ThreadLocal.ThreadLocalMap类型的变量 threadLocals 和 inheritableThreadLocals  
那我们再来看一下，ThreadLocal类中主要有这几个方法：

public void set(T value);
    public T get();
    private T setInitialValue();
    public void remove()

这几个方法主要定义了ThreadLocal设置，获取，设置初始值，和清除的方法。  
然后还可以看到ThreadLocal类中有一个静态内部类：ThreadLocalMap  
ThreadLocalMap内部其实就维护了一个Entry实体，大概跟HashMap差不多。  
对于ThreadLocal有一个理解上的误区就是我们会认为每个线程的变量副本存放到ThreadLocal里面，这么理解是不对的，应该是每个线程的副本存放到了ThreadLocal.ThreadLocalMap类型的一个容器里面，这个容器跟Map类似其实是一个Entry。下面我们来看一下ThreadLocalMap的源码：

static class ThreadLocalMap { 
    
            /** * The entries in this hash map extend WeakReference, using * its main ref field as the key (which is always a * ThreadLocal object). Note that null keys (i.e. entry.get() * == null) mean that the key is no longer referenced, so the * entry can be expunged from table. Such entries are referred to * as "stale entries" in the code that follows. */
            static class Entry extends WeakReference<ThreadLocal<?>> { 
                /** The value associated with this ThreadLocal. */
                Object value;
    
                Entry(ThreadLocal<?> k, Object v) { 
                    super(k);
                    value = v;
                }
            }
        }

这里我省去了很多内容，由上可以ThreadLocalMap是ThreadLocal的一个静态内部类，他的构成主要由Entry来保存数据，而且还是继承的WeakReference（弱引用），这里会给后面一个ThreadLocal的变量容易引起内存泄漏的问题买了一个伏笔，这里暂且不说。  
好了，现在我们知道了ThreadLocal修饰的变量其实是存放在一个ThreadLocal.ThreadLocalMap类型的一个Entry实体里面的。那么解析来我们再来看一下ThreadLocal中的

public void set(T value);
    public T get();
    private T setInitialValue();
    public void remove()

的这几个方法。

**3.1ThreadLocal的set方法：**

public void set(T value) { 
    		// 1.获取当前线程
            Thread t = Thread.currentThread();
            // 2.以当前线程 为key去查找对应的线程变量，找到对应的value
            ThreadLocalMap map = getMap(t);
            // 3.如果map不为空，则直接添加本地变量，key为当前定义的ThreadLocal变量的this引用，值为添加的本地变量的值
            if (map != null)
                map.set(this, value);
            else
            // 4.如果map为空，说明是首次添加，需要先创建出对应的map,并且key为当前定义的ThreadLocal变量的引用，value作为添加的本地变量的值
                createMap(t, value);
        }

getMap和createMap方法如下：

ThreadLocalMap getMap(Thread t) { 
            return t.threadLocals;
        }
    void createMap(Thread t, T firstValue) { 
            t.threadLocals = new ThreadLocalMap(this, firstValue);
        }

**3.2ThreadLocal的get方法：**

public T get() { 
    		// 1.获取当前线程
            Thread t = Thread.currentThread();
            // 2.根据当前线程或者threadLocals变量
            ThreadLocalMap map = getMap(t);
            // 3.如果获取到的threadLocals变量不为空，就可以直接拿到本地变量的值
            if (map != null) { 
                ThreadLocalMap.Entry e = map.getEntry(this);
                if (e != null) { 
                    @SuppressWarnings("unchecked")
                    T result = (T)e.value;
                    return result;
                }
            }
            // 4.如果运行到这里，说明没有获取到threadLocals变量，则调用初始化当前线程threadLocals变量的方法
            return setInitialValue();
        }

private T setInitialValue() { 
            T value = initialValue();
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);
            if (map != null)
                map.set(this, value);
            else
                createMap(t, value);
            return value;
        }
    protected T initialValue() { 
            return null;
        }
        void createMap(Thread t, T firstValue) { 
            t.threadLocals = new ThreadLocalMap(this, firstValue);
        }

接上面，由上可以看到初始化threalLocals变量时会初始化一个null,这就是为什么上面第二块代码中第一次调用get方法是会得到一个null的原因。

**3.3 ThreadLocal的setInitialValue()方法**  
继续粘一下setInitialValue()方法

private T setInitialValue() { 
    		// 1.获取一个初始化的value，这个初始化的value为null
            T value = initialValue();
            // 2.获取当前线程
            Thread t = Thread.currentThread();
            // 根据当前线程获取threalLocals变量
            ThreadLocalMap map = getMap(t);
            // 如果能获取到threadLocals变量，则直接设置该值给这个threadLocal变量
            if (map != null)
                map.set(this, value);
            // 如果没有获取到threadLocals变量,则初始化一个map,并把该值设置进去
            else
                createMap(t, value);
            return value;
        }
    protected T initialValue() { 
            return null;
        }
        void createMap(Thread t, T firstValue) { 
            t.threadLocals = new ThreadLocalMap(this, firstValue);
        }

**3.4ThreadLocal的remove()方法**

public void remove() { 
    		// 1.获取当前线程绑定的threadLocals变量
             ThreadLocalMap m = getMap(Thread.currentThread());
             // 2.如果获取到的map不为空，则移除当前线程中指定的ThreadLocal实例的本地变量
             if (m != null)
                 m.remove(this);
         }

这里说一下，我们已经知道了，ThreadLocal修饰的变量每个线程都会复制一个副本存放到自己线程的Entry中，那么ThreadLocal的变量的生命周期和该线程一样，所以ThreadLocal的变量会一直存在，除非线程消亡了，ThreadLocal修饰的变量才会被回收，那么就容易会造成内存泄漏，所以在线程中对于ThreadLocal的变量，如果用完了就最好手动remove()一下。

**四。ThreadLocal的实际应用**

public class ThreadLocal5 { 
    
        static ThreadLocal<Map<String, String>> localMap = new ThreadLocal<Map<String, String>>(){ 
            @Override
            protected Map<String, String> initialValue() { 
                Map<String, String> map = new HashMap<>(2);
                map.put("startTime", "2020-01-01");
                map.put("endTime", "2021-12-01");
                return map;
            }
        };
    
    
    
        static void print(Map<String, String> map){ 
            for (Map.Entry<String, String> entry : map.entrySet()) { 
                System.out.println(entry.getKey()+":"+entry.getValue());
            }
        }
    
        public static void main(String[] args) { 
            System.out.println("main-start");
    
            Thread thread1 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread1-start...");
                    System.out.println("thread1-start-第一次输出完了...");
                    Map<String, String> map = localMap.get();
                    map.put("date", "2021-02-01");
                    map.put("startTime", "2020-07-01");
                    localMap.set(map);
                    print(localMap.get());
                    System.out.println("thread1-end...");
                }
            });
    
            Thread thread2 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread2-start...");
                    System.out.println("thread2-start-第一次输出完了...");
                    Map<String, String> map = localMap.get();
                    map.put("startTime", "2020-08-01");
                    map.put("seq", "12");
                    localMap.set(map);
                    print(localMap.get());
                    System.out.println("thread2-end...");
                }
            });
    
            thread1.start();
            thread2.start();
    
            System.out.println("main-end");
        }
    }

这里是我写的一个ThreadLocal的map,并且初始化了其中的值，每个变量都会共享localMap变量。

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IDEA.app/Contents/lib/idea_rt.jar com.taojiang.threaddemo.threadlocal.ThreadLocal5
    Connected to the target VM, address: '127.0.0.1:60397', transport: 'socket'
    main-start
    thread1-start...
    thread1-start-第一次输出完了...
    main-end
    thread2-start...
    thread2-start-第一次输出完了...
    date:2021-02-01
    startTime:2020-08-01
    startTime:2020-07-01
    seq:12
    endTime:2021-12-01
    endTime:2021-12-01
    thread2-end...
    thread1-end...
    Disconnected from the target VM, address: '127.0.0.1:60397', transport: 'socket'
    
    Process finished with exit code 0

由上可知每个线程可以随便使用localMap变量，随意修改后不会影响别的线程。

**五。ThreadLocal不支持继承性**  
同一个ThrealLocal变量在父线程中被设置了值后，在子线程中是获取不到的  
代码实例如下：

public class ThreadLocal6 { 
    
        static ThreadLocal<String> localVar = new ThreadLocal<>();
    
    
        public static void main(String[] args) { 
            System.out.println("main-start");
    
            localVar.set("init");
    
            Thread thread1 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread1-start...");
                    System.out.println("thread1设置前:"+localVar.get());
                    localVar.set("localVar01");
                    System.out.println("thread1设置后:"+localVar.get());
                    System.out.println("thread1-end...");
                }
            });
    
            Thread thread2 = new Thread(new Runnable() { 
                @Override
                public void run() { 
                    System.out.println("thread2-start...");
                    System.out.println("thread2设置前:"+localVar.get());
                    localVar.set("localVar02");
                    System.out.println("thread2设置后:"+localVar.get());
                    System.out.println("thread2-end...");
                }
            });
    
            thread1.start();
            thread2.start();
            System.out.println("main-getlocalVar:"+localVar.get());
    
            System.out.println("main-end");
        }
    }

运行结果如下：

main-start
    thread1-start...
    thread1设置前:null
    thread1设置后:localVar01
    thread1-end...
    thread2-start...
    thread2设置前:null
    thread2设置后:localVar02
    thread2-end...
    main-getlocalVar:init
    main-end

由此可以看出子线程是拿不到父线程事先设置的ThrealLocal变量的值的。所以ThreadLocal不具有继承性。

**六。InheritableThreadLocal类**  
上面说道ThreadLocal类不能继承，那么我们可以做个父传子吗？答案是：可以，使用InheritableThreadLocal类就可以。  
具体为什么可以，源码这里不展开说了。  
今天先写到这里。写的有点乱，如果有错误欢迎大家指正。

**七。ThreadLocal和同步方法解决线程安全问题的区别**

学习完本节我们知道了在某些场景下ThrealLocal也可以保证多线程线程安全，那么他跟同步方法有什么区别呢？  
同步方法是牺牲了时间，也就是在同一时刻只能有一个线程访问共享变量，所以同步方法是时间换安全的一种方式  
而ThrealLocal是每一个线程都有一个共享变量的副本，线程操作的是自己的那份拷贝，所以也解决了线程安全的问题，这种方式是以空间换安全的做法。