Threadlocal 特性 1.线程间共享变量:一个线程在设置threadlocal值之后,当前线程无论在哪里都可以从设置的threadlocal中获取值,解决值传递的问题。 2.不同线程间隔离:线程a在设置threadlocal值之后,线程b是无法获取线程a设置的值,解决了线程安全的问题。 Threadlocal 简单使用
public class ThreadLocalTest { private static ThreadLocal<String> fruit = new ThreadLocal<>(); public static void setFruit(String param) { fruit.set(param); } public static String getFruit() { return fruit.get(); } public static void main(String[] args) { MyThread1 thread1 = new MyThread1(); MyThread2 thread2 = new MyThread2(); thread1.start(); thread2.start(); } public static class MyThread1 extends Thread { @Override public void run() { try { setFruit("apple"); System.out.println("MyThread1 线程设置的水果:apple"); sleep(3000); System.out.println("MyThread1 线程获取的水果:"+getFruit()); } catch (InterruptedException e) { e.printStackTrace(); } } } public static class MyThread2 extends Thread { @Override public void run() { try { sleep(1000); setFruit("orange"); System.out.println("MyThread2 线程设置的水果:orange"); sleep(1000); System.out.println("MyThread2 线程获取的水果:"+getFruit()); } catch (InterruptedException e) { e.printStackTrace(); } } } }
程序运行打印出结果
MyThread1 线程设置的水果:apple MyThread2 线程设置的水果:orange MyThread2 线程获取的水果:orange MyThread1 线程获取的水果:apple由此可见之前叙述的两种特性。 深入threadlocal学习,拥有这两种特性的原因。 ThreadLocal.class set方法
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocal.ThreadLocalMap map = this.getMap(t); if (map != null) { map.set(this, value); } else { this.createMap(t, value); } }
ThreadLocalMap getMap(Thread t) { return t.threadLocals; }
在使用set方法时,会获取到当前thread对象的引用,而this.getMap方法则是从当前对象中获取threadLocals参数,而此参数的类型为ThreadLocalMap,通过<key,value>的方式存储数据
此时key:当前threadlocal的引用。value则是要存储的数据 1.当前map 为空时,调用 this.createMap(t, value)void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }
看下ThreadLocalMap
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) { table = new Entry[INITIAL_CAPACITY]; int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); table[i] = new Entry(firstKey, firstValue); size = 1; setThreshold(INITIAL_CAPACITY); }2.map中有数据,看下 map.set(this, value) 方法里具体操作。
private void set(ThreadLocal<?> key, Object value) { Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal<?> k = e.get(); if (k == key) { e.value = value; return; } if (k == null) { replaceStaleEntry(key, value, i); return; } } tab[i] = new Entry(key, value); int sz = ++size; if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); }
综上两个方法可以看出threadlocal中处理key ,value的是用Entry 去接收的。
看下Entry.classstatic class Entry extends WeakReference<ThreadLocal<?>> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal<?> k, Object v) { super(k); value = v; } }Entry是继承WeakReference,说明Entry 这边是一个虚引用,而虚引用在垃圾回收器线程扫描它所管辖的内存区域的过程中,一旦发现了只具有弱引用的对象,不管当前内存空间足够与否,都会回收它的内存。 get方法
public T get() { Thread t = Thread.currentThread(); 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(); }
remove方法
public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); }
值得注意的是,在结合set,get 使用后,需要结合remove方法讲堆内存中的对象释放掉,不然会造成内存泄露。
下面看下内存图标签:Thread,value,ThreadLocal,线程,Entry,public,作用 From: https://www.cnblogs.com/lingyujuan/p/17052404.html