并发问题解决小结

关于一次自动转单出现的并发问题的解决：

1.方案：直接用synchronized(customerId),

存在问题：可能存在坑，字符串的hashCode相同时地址不一定相同，详情见博客：https://blog.csdn.net/u014653197/article/details/76177277

详见代码：

package autoTransfer;

import java.util.Scanner;

/**
 * Created by cuijunyong on 2018/6/27.
 */
public class TestMain {
  private static AutoTransfer autoTransfer = new AutoTransfer();
  private static Scanner cin = new Scanner(System.in);
  public static void main(String[] args) {
//    String a = cin.next();
//    String b = cin.next();
//    String c = cin.next();

    new MyThread("123").start();
    new MyThread("123").start();
    new MyThread(new String("123")).start();

  }


}

class MyThread extends Thread{

  private String customerId;
  private static AutoTransfer autoTransfer = new AutoTransfer();
  @Override
  public void run() {
    try {
      autoTransfer.run(customerId);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  public MyThread(String customerId) {
    this.customerId = customerId;
  }
}

View Code

package autoTransfer;

/**
 * Created by cuijunyong on 2018/6/27.
 */
public class AutoTransfer{

  public void run(String customerId) throws Exception{
    synchronized (customerId){
      System.out.println(Thread.currentThread() + "自动转单开始customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正内存地址=" + System.identityHashCode(customerId));
      Thread.sleep(5000);
      System.out.println(Thread.currentThread() + "自动转单结束customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正内存地址=" + System.identityHashCode(customerId));
    }
  }

}

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这个简单一点的解决方案可以直接synchronized(customerId.intern())

2.方案：用concurrentHashMap 进入用map.put(customerId)的返回值判断是否存在，如果返回值不为空，证明已经有customerId在访问，就不执行方法。方法结束时remove掉

存在问题：如果中途抛出异常会不会造成这一cuntomerId一直不会被remove掉。

3.方案：添加try{

    if(map.put(customerId)!=null)return;

    Dosomething();

}catch(){

}

finally{

    map.remove(customerId);

}finally内remove掉

存在问题：map.put(customerId)放在了try里，会造成并发访问return掉的也执行了finally方法，下次并行就还会造成并发问题。

4.方案：于是将if(map.put(customerId)!=null)return;放到try外边

存在问题：出现异常不能被抛出去，前端获取不到异常。

5.解决方案：不用catch了直接try{}finally{} 

package autoTransfer;

import java.util.Scanner;

/**
 * Created by cuijunyong on 2018/6/27.
 */
public class TestMain {
  private static AutoTransfer autoTransfer = new AutoTransfer();
  private static Scanner cin = new Scanner(System.in);
  public static void main(String[] args) {
//    String a = cin.next();
//    String b = cin.next();
//    String c = cin.next();

    new MyThread("123").start();
    new MyThread("123").start();
    new MyThread(new String("123")).start();

  }


}

class MyThread extends Thread{

  private String customerId;
  private static AutoTransfer autoTransfer = new AutoTransfer();
  @Override
  public void run() {
    try {
      autoTransfer.run(customerId);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  public MyThread(String customerId) {
    this.customerId = customerId;
  }
}

View Code

package autoTransfer;

import java.util.concurrent.ConcurrentHashMap;

/**
 * Created by cuijunyong on 2018/6/27.
 */
public class AutoTransfer{

  private ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
  public void run(String customerId) throws Exception{
    if(map.put(customerId, 1) != null){
      return;
    }
    try {
      System.out.println(Thread.currentThread() + "自动转单开始customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正内存地址=" + System.identityHashCode(customerId));
      Thread.sleep(5000);
      System.out.println(Thread.currentThread() + "自动转单结束customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正内存地址=" + System.identityHashCode(customerId));
    }finally {
      map.remove(customerId);
    }

  }

}

View Code