Java并发引入之优缺点

并发分为“进程并发”和“线程并发”，Java并发属于后者。

一、优点

• 使能充分利用现代计算机的多CPU和多CPU核，提升并行/并发度，加快程序执行速度
• 契合某些问题域模型，比如“生产者-消费者问题”

二、缺点

• 需要耗费额外的资源，比如“线程创建”，“用于保存线程对象的内存”，“因上下文切换而消耗的CPU时间”等
• 需要耗费更多精力用于编写正确可运行，线程安全的程序，需要考虑“可见性，有序性，原子性，死锁”等问题

以下是一份简单的会导致出现死锁的示例代码：

public class DeadLockDemo {
    private static String A = "A";
    private static String B = "B";

    public static void main(String[] args) {
        new DeadLockDemo().deadLock();
    }

    private void deadLock() {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (A) {
                    try {
                        Thread.currentThread().sleep(2000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    synchronized (B) {
                        System.out.println("1");
                    }
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (B) {
                    synchronized (A) {
                        System.out.println("2");
                    }
                }
            }
        });

        t1.start();
        t2.start();
    }

}

执行该Java程序，通过jstack命令查看线程栈，以下是呈现死锁状态的线程栈相关内容：

"Thread-1" #9 prio=5 os_prio=0 tid=0x00007f34d80cd000 nid=0x513c waiting for monitor entry [0x00007f34dcb69000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at DeadLockDemo$2.run(DeadLockDemo.java:33)
	- waiting to lock <0x00000000b2e5a7e8> (a java.lang.String)
	- locked <0x00000000b2e5a818> (a java.lang.String)
	at java.lang.Thread.run(Thread.java:748)

"Thread-0" #8 prio=5 os_prio=0 tid=0x00007f34d80cb800 nid=0x513b waiting for monitor entry [0x00007f34dcc6a000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at DeadLockDemo$1.run(DeadLockDemo.java:21)
	- waiting to lock <0x00000000b2e5a818> (a java.lang.String)
	- locked <0x00000000b2e5a7e8> (a java.lang.String)
	at java.lang.Thread.run(Thread.java:748)