併發問題解決小結

關於一次自動轉單出現的併發問題的解決：

1.方案：直接用synchronized(customerId),

存在問題：可能存在坑，字符串的hashCode相同時地址不必定相同，詳情見博客：https://blog.csdn.net/u014653197/article/details/76177277

詳見代碼：

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() + "自動轉單開始customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正內存地址=" + System.identityHashCode(customerId));
      Thread.sleep(5000);
      System.out.println(Thread.currentThread() + "自動轉單結束customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正內存地址=" + System.identityHashCode(customerId));
    }
  }

}

View Code

這個簡單一點的解決方案能夠直接synchronized(customerId.intern())

2.方案：用concurrentHashMap 進入用map.put(customerId)的返回值判斷是否存在，若是返回值不爲空，證實已經有customerId在訪問，就不執行方法。方法結束時remove掉

存在問題：若是中途拋出異常會不會形成這一cuntomerId一直不會被remove掉。

3.方案：添加try{

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

    Dosomething();

}catch(){

}

finally{

    map.remove(customerId);

}finally內remove掉

存在問題：map.put(customerId)放在了try裏，會形成併發訪問return掉的也執行了finally方法，下次並行就還會形成併發問題。

4.方案：因而將if(map.put(customerId)!=null)return;放到try外邊

存在問題：出現異常不能被拋出去，前端獲取不到異常。

5.解決方案：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() + "自動轉單開始customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正內存地址=" + System.identityHashCode(customerId));
      Thread.sleep(5000);
      System.out.println(Thread.currentThread() + "自動轉單結束customerId=" + customerId + " hashCode=" + customerId.hashCode() + "真正內存地址=" + System.identityHashCode(customerId));
    }finally {
      map.remove(customerId);
    }

  }

}

View Code