并发编程--并发编程包LockSupport

素颜马尾好姑娘i 2022-06-18 11:15 241阅读 0赞

LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。LockSupport主要提供了两个功能：

（1）park()方法，用来阻塞线程。

（2）unpark()方法，解除阻塞线程。

LockSupport提供的park()和unpark()方法不会遇到Thread.suspend和Thread.resume所可能引起的死锁问题，因为park和unpark有许可的存在，调用park的线程和另一个试图将其unpark()的线程之间的竞争将保持活性。

LockSupport的实现基础是通过sun.misc.Unsafe(真是一个功能强大的类)，通过Unsafe提供的park()和unpark()方法来完成线程的阻塞和解除阻塞。

park()阻塞线程：

public static void park(Object blocker) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            UNSAFE.park(false, 0L);
            setBlocker(t, null);
        }

private static void setBlocker(Thread t, Object arg) {
            // Even though volatile, hotspot doesn't need a write barrier here.
            UNSAFE.putObject(t, parkBlockerOffset, arg);
        }

完成线程的阻塞，此时线程处于阻塞状态。

unpark()解除线程阻塞：

public static void unpark(Thread thread) {
            if (thread != null)
                UNSAFE.unpark(thread);
        }

完成线程thread的阻塞，这样线程处于可执行状态。

简单示例：子线程运行完之后主线程才运行结束

public class LockSupportLean {
    
    	private static Thread mainThread;
    	
    	public static void main(String[] args) {
    
    			mainThread = Thread.currentThread();
    			ThreadA threadA = new ThreadA("a");
    			System.err.println("阻塞主线程");
    			threadA.start();
    			LockSupport.park(mainThread);
    	}
    	static class ThreadA extends Thread{
    		
    		public ThreadA(String name){
    			super(name);
    		}
    		
    		@Override
    		public void run(){
    			System.err.println("运行子线程");
    			LockSupport.unpark(mainThread);
    		}
    		
    	}
    
    }

LockSupport源码（jdk8）

public class LockSupport {
        private LockSupport() {} // Cannot be instantiated.
    
        private static void setBlocker(Thread t, Object arg) {
            // Even though volatile, hotspot doesn't need a write barrier here.
            UNSAFE.putObject(t, parkBlockerOffset, arg);
        }
    
        
        public static void unpark(Thread thread) {
            if (thread != null)
                UNSAFE.unpark(thread);
        }
    
        public static void park(Object blocker) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            UNSAFE.park(false, 0L);
            setBlocker(t, null);
        }
    
        public static void parkNanos(Object blocker, long nanos) {
            if (nanos > 0) {
                Thread t = Thread.currentThread();
                setBlocker(t, blocker);
                UNSAFE.park(false, nanos);
                setBlocker(t, null);
            }
        }
    
        public static void parkUntil(Object blocker, long deadline) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            UNSAFE.park(true, deadline);
            setBlocker(t, null);
        }
    
        public static Object getBlocker(Thread t) {
            if (t == null)
                throw new NullPointerException();
            return UNSAFE.getObjectVolatile(t, parkBlockerOffset);
        }
    
        public static void park() {
            UNSAFE.park(false, 0L);
        }
    
       
        public static void parkNanos(long nanos) {
            if (nanos > 0)
                UNSAFE.park(false, nanos);
        }
    
       
        public static void parkUntil(long deadline) {
            UNSAFE.park(true, deadline);
        }
    
        static final int nextSecondarySeed() {
            int r;
            Thread t = Thread.currentThread();
            if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {
                r ^= r << 13;   // xorshift
                r ^= r >>> 17;
                r ^= r << 5;
            }
            else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
                r = 1; // avoid zero
            UNSAFE.putInt(t, SECONDARY, r);
            return r;
        }
    
       
        private static final sun.misc.Unsafe UNSAFE;
        private static final long parkBlockerOffset;
        private static final long SEED;
        private static final long PROBE;
        private static final long SECONDARY;
        static {
            try {
                UNSAFE = sun.misc.Unsafe.getUnsafe();
                Class<?> tk = Thread.class;
                parkBlockerOffset = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("parkBlocker"));
                SEED = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomSeed"));
                PROBE = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomProbe"));
                SECONDARY = UNSAFE.objectFieldOffset
                    (tk.getDeclaredField("threadLocalRandomSecondarySeed"));
            } catch (Exception ex) { throw new Error(ex); }
        }
    
    }