共享受限资源及锁

[TOC]

1.不正确的访问资源

懒汉单例模式，线程不安全

public class SingletonDemo1 {
    private static SingletonDemo1 instance;
    private SingletonDemo1(){}
    public static SingletonDemo1 getInstance(){
        if (instance == null) {
            instance = new SingletonDemo1();
        }
        return instance;
    }
}

2.锁(synchronized)

public class SingletonDemo2 {
    private static SingletonDemo2 instance;
    private SingletonDemo2(){}
    public static synchronized SingletonDemo2 getInstance(){
        if (instance == null) {
            instance = new SingletonDemo2();
        }
        return instance;
    }
}

3.显式的锁(Lock)

import java.util.concurrent.locks.*;

public class SingletonDemo2 {
    private static SingletonDemo2 instance;
    private SingletonDemo2(){}
    private static Lock lock = new ReentrantLock();
    public static SingletonDemo2 getInstance(){
        lock.lock();
        try{
            if (instance == null) {
                instance = new SingletonDemo2();
            }
            return instance; 
        } finally {
            lock.unlock();
        }
    }
}

用synchronized不能尝试着获取锁且最终获取锁会失败，或尝试获取锁一段时间，然后放弃它时，应使用Lock锁：

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.*;

public class SingletonDemo2 {
    private static SingletonDemo2 instance;
    private SingletonDemo2(){}
    private static Lock lock = new ReentrantLock();
    public static SingletonDemo2 getInstance(){
        boolean captured = false;
        try {
            captured = lock.tryLock(2, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        lock.lock();
        try{
            if (instance == null) {
                instance = new SingletonDemo2();
            }
            return instance;
        } finally {
            if (captured){
                lock.unlock();
            }
        }
    }
}

4.原子性

原子性操作是不能被线程调度机制中断的操作。原子性可应用于除long 和 double 之外的所有基本数据类型，因jvm会将64位的long和double的读取和写入当做两个分离的32位操作，从而产生“字撕裂”。如果使用volatile关键字，可以使long和double 获得原子性。

java 中仅基本数据类型（除long和double）的读取和赋值为院子操作。

public class Atomicity {
  int i;
  void f1() { i++; }
  void f2() { i += 3; }
} /* Output: (Sample)
...
void f1();
  Code:
   0:        aload_0
   1:        dup
   2:        getfield        #2; //Field i:I
   5:        iconst_1
   6:        iadd
   7:        putfield        #2; //Field i:I
   10:        return

void f2();
  Code:
   0:        aload_0
   1:        dup
   2:        getfield        #2; //Field i:I
   5:        iconst_3
   6:        iadd
   7:        putfield        #2; //Field i:I
   10:        return
*///:~

每条指令都会产生一个get和put，它们之间还有其他指令。

5.原子类

AtomicInteger

AtomicLong

AtomicReference

6.临界区

synchronized被用来指定某个对象，此对象的锁被用来对花括号内的代码进行同步控制：

synchronized(syncObject){
     statements;
}

7.线程本地存储 (ThreadLocal)

线程本地存储是一种自动化的机制，可以为使用相同变量的每个线程都创建不同的存储。

//: ThreadLocalVariableHolder.java
// Automatically giving each thread its own storage.
import java.util.concurrent.*;
import java.util.*;

class Accessor implements Runnable {
  private final int id;
  public Accessor(int idn) { id = idn; }
  public void run() {
    while(!Thread.currentThread().isInterrupted()) {
      ThreadLocalVariableHolder.increment();
      System.out.println(this);
      Thread.yield();
    }
  }
  public String toString() {
    return "#" + id + ": " +
      ThreadLocalVariableHolder.get();
  }
}

public class ThreadLocalVariableHolder {
  private static ThreadLocal value =
    new ThreadLocal() {
      private Random rand = new Random(47);
      protected synchronized Integer initialValue() {
        return rand.nextInt(10000);
      }
    };
  public static void increment() {
    value.set(value.get() + 1);
  }
  public static int get() { return value.get(); }
  public static void main(String[] args) throws Exception {
    ExecutorService exec = Executors.newCachedThreadPool();
    for(int i = 0; i < 5; i++)
      exec.execute(new Accessor(i));
    TimeUnit.SECONDS.sleep(3);  // Run for a while
    exec.shutdownNow();         // All Accessors will quit
  }
} /* Output: (Sample)
#0: 9259
#1: 556
#2: 6694
#3: 1862
#4: 962
#0: 9260
#1: 557
#2: 6695
#3: 1863
#4: 963
...
*///:~

8.在阻塞时终结(中断)

线程状态：

1. 新建
2. 就绪
3. 阻塞
4. 死亡

进入阻塞状态的原因：

1. 调用sleep()使任务休眠
2. 调用wait()使任务挂起
3. 任务在等待某个输入/输出完成
4. 任务在等锁

中断任务可调用Thread.interrupted()或 调用executor的shutdownNow()方法，会发送一个interrupted()调用给他启动的所有线程。如果使用Executor的submit()来启动任务，就可以持有该任务的上下文。submit()将返回一个Future，可以调用其cancel()来只中断某个特定的任务。

其中仅sleep状态可中断。等待io和等待锁，都会使任务的中断失效：

解决：

1.关闭任务在其上发生阻塞的底层资源。

2.interrupt()可打断被互斥阻塞的调用