1.業務場景簡介

    儲存使用者個人設定，一個使用者在a表中只能存在一條記錄，使用者修改記錄時，檢查如果該記錄存在，則修改該資料，如果該記錄不存在，則插入一條記錄。

2. 問題說明

    多執行緒情況下，會出現一個使用者在表中有多條記錄存在的情況。

3. 原因解釋

    比如說併發兩個執行緒A和執行緒B， 執行緒A檢查記錄不存在，準備插入記錄，這時CPU切換到執行緒B，這是執行緒B檢查記錄仍是不存在，執行緒B插入一條資料，執行緒B結束。然後CPU切換到執行緒A，A繼續插入記錄，這是資料庫就會有兩條記錄。

package com.salen.test;

import java.util.concurrent.atomic.AtomicInteger;

public class MultiThreadTest {

	//計數器，記錄插入次數
	static AtomicInteger num = new AtomicInteger(0);
	
	//記錄是否存在， 模擬資料庫記錄，表示某條資訊是否存在
	static boolean exist = false;
	
	public static void main(String[] args) {
		
		//起20個執行緒，同時執行檢查插入操作
		for (int i = 0; i < 20; i++) {
			new Thread(new Runnable() {
				@Override
				public void run() {
					try {
						MultiThreadTest test = new MultiThreadTest();
						test.generate();
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}).start();
		}
		
	}
	
	
	/**
	 * 模擬修改一條資料
	 * 		先去檢查該資料是否存在，
	 * 		如果該記錄存在，則修改該資料，如果資料不存在，則插入一條新的資料
	 * @throws InterruptedException 
	 */
	private void generate() throws InterruptedException{
		
		if(exist){
			Thread.sleep(1000); // 處理業務 100毫秒
			System.out.println("record has exist..., update it ");
		}else{
			Thread.sleep(100); // 處理業務 100毫秒
			System.out.println("record not exist..., insert into it, time = " + num.incrementAndGet());
			exist = true;
		}
		
	}
	
}

 

測試結果

record not exist..., insert into it, time = 2
record not exist..., insert into it, time = 3
record not exist..., insert into it, time = 1
record not exist..., insert into it, time = 4
record not exist..., insert into it, time = 5
record not exist..., insert into it, time = 6
record not exist..., insert into it, time = 7
record not exist..., insert into it, time = 8
record not exist..., insert into it, time = 10
record not exist..., insert into it, time = 9
record not exist..., insert into it, time = 11
record not exist..., insert into it, time = 12
record not exist..., insert into it, time = 13
record not exist..., insert into it, time = 14
record not exist..., insert into it, time = 15
record not exist..., insert into it, time = 17
record not exist..., insert into it, time = 16
record not exist..., insert into it, time = 18
record not exist..., insert into it, time = 19
record not exist..., insert into it, time = 20