Java模擬并解決緩存穿透問題

更新時間：2019年08月08日 10:51:42 作者：mqk100

這篇文章主要介紹了Java模擬并解決緩存穿透問題，本文給大家介紹的非常詳細(xì)，具有一定的參考借鑒價值 ,需要的朋友可以參考下

什么叫做緩存穿透

緩存穿透只會發(fā)生在高并發(fā)的時候，就是當(dāng)有10000個并發(fā)進(jìn)行查詢數(shù)據(jù)的時候，我們一般都會先去redis里面查詢進(jìn)行數(shù)據(jù)，但是如果redis里面沒有這個數(shù)據(jù)的時候，那么這10000個并發(fā)里面就會有很大一部分并發(fā)會一下子都去mysql數(shù)據(jù)庫里面進(jìn)行查詢了

解決緩存穿透

首先我模擬一下緩存穿透

比如下面的代碼

Pom.xml代碼

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.example</groupId>
 <artifactId>springboot</artifactId>
 <version>0.0.1-SNAPSHOT</version>
 <packaging>jar</packaging>
 <name>springboot</name>
 <description>Demo project for Spring Boot</description>
 <parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.1.1.RELEASE</version>
  <relativePath></relativePath> <!-- lookup parent from repository -->
 </parent>
 <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
  <java.version>1.8</java.version>
 </properties>
 <dependencies>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-devtools</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>org.mybatis.spring.boot</groupId>
   <artifactId>mybatis-spring-boot-starter</artifactId>
   <version>1.1.1</version>
  </dependency>
  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
  </dependency>
 </dependencies>
 <build>
  <plugins>
   <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
   </plugin>
  </plugins>
 </build>
</project>
Application.properties
server.port=8081
#DB Configuration:
spring.datasource.driverClassName=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://47.91.248.236:3306/hello?useUnicode=true&characterEncoding=utf8
spring.datasource.username=root
spring.datasource.password=root
#spring集成Mybatis環(huán)境
#pojo別名掃描包
mybatis.type-aliases-package=com.itheima.domain
#加載Mybatis映射文件
mybatis.mapper-locations=classpath:mapper/*Mapper.xml
MyController代碼，下面的藍(lán)色代碼是模仿10000個并發(fā)線程
/**
 * sinture.com Inc.
 * Copyright (c) 2016-2018 All Rights Reserved.
 */
package com.itheima.controller;
import com.itheima.mapper.UserMapper;
import com.itheima.domain.User;
import com.itheima.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.RestController;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
/**
 * @author xinzhu
 * @version Id: MyController.java, v 0.1 2018年12月05日 下午6:29 xinzhu Exp $
 */
@RestController
public class MyController {
 @Autowired
 private UserService userService;
 @RequestMapping("/hello/{id}")
 @ResponseBody
 public User queryUser(@PathVariable Integer id){
  // 藍(lán)色代碼注釋開始
  new Thread(){
   @Override
   public void run() {
    for (int x = 0; x < 10000; x++) {
     userService.queryUser(id);
    }
   }
  }.start();
  // 藍(lán)色代碼注釋結(jié)束
  return userService.queryUser(id);
 }
}

User類

/**
 * sinture.com Inc.
 * Copyright (c) 2016-2018 All Rights Reserved.
 */
package com.itheima.domain;
/**
 * @author xinzhu
 * @version Id: User.java, v 0.1 2018年12月06日 下午1:40 xinzhu Exp $
 */
public class User {
 // 主鍵
 private Integer id;
 // 用戶名
 private String username;
 // 密碼
 private String password;
 // 姓名
 private String name;
 public void setId(Integer id) {
  this.id = id;
 }
 @Override
 public String toString() {
  return "User{" +
    "id=" + id +
    ", username='" + username + '\'' +
    ", password='" + password + '\'' +
    ", name='" + name + '\'' +
    '}';
 }
 public Integer getId() {
  return id;
 }
 public String getUsername() {
  return username;
 }
 public void setUsername(String username) {
  this.username = username;
 }
 public String getPassword() {
  return password;
 }
 public void setPassword(String password) {
  this.password = password;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
}
UserService
package com.itheima.service;
import com.itheima.domain.User;
public interface UserService {
 public User queryUser(Integer id);
}

UserServiceImpl，下面的藍(lán)色代碼就是模仿redis，此時要注意下面的模擬redis的map集合必須放到下面的queryUser的外面，也就是說下面的userMap變量必須是成員變量，不然的話，因?yàn)閞edis是被多個線程共享的，如果你放到下面的queryUser()方法里面，那么就是多個線程有多個userMap集合，下面的代碼就是如果查詢到數(shù)據(jù)，那么就用redis里面的，如果查詢不到就用數(shù)據(jù)庫里面的

package com.itheima.service;
import com.itheima.domain.User;
import com.itheima.mapper.UserMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.HashMap;
import java.util.Map;
@Service
public class UserServiceImpl implements UserService {
 @Autowired
 private UserMapper userMapper;
 // 藍(lán)色代碼注釋開始
 static Map<Integer,User> userMap=new HashMap();
 static {
  User huancun_user =new User();
  huancun_user.setId(1);
  huancun_user.setName("zhangsan");
  huancun_user.setPassword("123");
  huancun_user.setName("張三");
  userMap.put(1,huancun_user);
 }
 // 藍(lán)色代碼注釋結(jié)束
 public User queryUser(Integer id){
  User user=userMap.get(id);
  if(user==null){
   user= userMapper.queryUser(id);
   System.out.println("查詢數(shù)據(jù)庫");
   userMap.put(user.getId(),user);
  }else{
   System.out.println("查詢緩存");
  }
  return user;
 };
}
SpringbootApplication代碼
package com.itheima;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class SpringbootApplication {
 public static void main(String[] args) {
  SpringApplication.run(SpringbootApplication.class, args);
 }
}

數(shù)據(jù)庫里面的數(shù)據(jù)如下

-- ----------------------------
-- Table structure for `user`
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `username` varchar(50) DEFAULT NULL,
 `password` varchar(50) DEFAULT NULL,
 `name` varchar(50) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES ('1', 'zhangsan', '123', '張三');
INSERT INTO `user` VALUES ('2', 'lisi', '123', '李四');

然后我們查詢下面的鏈接，因?yàn)榇藭r上面的模擬redis的map集合里面沒有id值是2的數(shù)據(jù)，所以此時都是查詢數(shù)據(jù)庫，你想這一下子10000并發(fā)過去，數(shù)據(jù)庫會有很大壓力的，

然后打印結(jié)果如下，就是打印了很多查詢數(shù)據(jù)庫和查詢緩存，此時也就說明10000個并發(fā)里面有很多去查詢了數(shù)據(jù)庫，這個是要避免的，至于為什么有查詢緩存的打印，因?yàn)槲覀儼巡樵兊臄?shù)據(jù)給放到模擬的redis里面了啊，所以剛開始的一大部分線程都是查詢數(shù)據(jù)庫，然后剩下的都是查詢模擬的redis緩存里面的數(shù)據(jù)

查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢數(shù)據(jù)庫
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存
查詢緩存

然后我們使用雙重檢測鎖來解決上面的緩存穿透

我們怎么解決緩存穿透呢，即使10000個并發(fā)過來，然后這10000個并發(fā)需要的數(shù)據(jù)在redis里面都沒有，那么我們應(yīng)該第一個線程查詢數(shù)據(jù)里面的數(shù)據(jù)，然后把這個數(shù)據(jù)給放到redis里面，然后剩下的9999個線程都到redis里面查詢，這樣就解決了緩存穿透，所以我們可以把上面的代碼變成下面這樣

比如下面的代碼

Pom.xml代碼

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.example</groupId>
 <artifactId>springboot</artifactId>
 <version>0.0.1-SNAPSHOT</version>
 <packaging>jar</packaging>
 <name>springboot</name>
 <description>Demo project for Spring Boot</description>
 <parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.1.1.RELEASE</version>
  <relativePath></relativePath> <!-- lookup parent from repository -->
 </parent>
 <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
  <java.version>1.8</java.version>
 </properties>
 <dependencies>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-devtools</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>org.mybatis.spring.boot</groupId>
   <artifactId>mybatis-spring-boot-starter</artifactId>
   <version>1.1.1</version>
  </dependency>
  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
  </dependency>
 </dependencies>
 <build>
  <plugins>
   <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
   </plugin>
  </plugins>
 </build>
</project>
Application.properties
server.port=8081
#DB Configuration:
spring.datasource.driverClassName=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://47.91.248.236:3306/hello?useUnicode=true&characterEncoding=utf8
spring.datasource.username=root
spring.datasource.password=root
#spring集成Mybatis環(huán)境
#pojo別名掃描包
mybatis.type-aliases-package=com.itheima.domain
#加載Mybatis映射文件
mybatis.mapper-locations=classpath:mapper/*Mapper.xml

MyController代碼，下面的藍(lán)色代碼是模仿10000個并發(fā)線程

/**
 * sinture.com Inc.
 * Copyright (c) 2016-2018 All Rights Reserved.
 */
package com.itheima.controller;
import com.itheima.mapper.UserMapper;
import com.itheima.domain.User;
import com.itheima.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.RestController;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
/**
 * @author xinzhu
 * @version Id: MyController.java, v 0.1 2018年12月05日 下午6:29 xinzhu Exp $
 */
@RestController
public class MyController {
 @Autowired
 private UserService userService;
 @RequestMapping("/hello/{id}")
 @ResponseBody
 public User queryUser(@PathVariable Integer id){
  // 藍(lán)色代碼注釋開始
  new Thread(){
   @Override
   public void run() {
    for (int x = 0; x < 10000; x++) {
     userService.queryUser(id);
    }
   }
  }.start();
  // 藍(lán)色代碼注釋結(jié)束
  return userService.queryUser(id);
 }
}
User類
/**
 * sinture.com Inc.
 * Copyright (c) 2016-2018 All Rights Reserved.
 */
package com.itheima.domain;
/**
 * @author xinzhu
 * @version Id: User.java, v 0.1 2018年12月06日 下午1:40 xinzhu Exp $
 */
public class User {
 // 主鍵
 private Integer id;
 // 用戶名
 private String username;
 // 密碼
 private String password;
 // 姓名
 private String name;
 public void setId(Integer id) {
  this.id = id;
 }
 @Override
 public String toString() {
  return "User{" +
    "id=" + id +
    ", username='" + username + '\'' +
    ", password='" + password + '\'' +
    ", name='" + name + '\'' +
    '}';
 }
 public Integer getId() {
  return id;
 }
 public String getUsername() {
  return username;
 }
 public void setUsername(String username) {
  this.username = username;
 }
 public String getPassword() {
  return password;
 }
 public void setPassword(String password) {
  this.password = password;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
}
UserService
package com.itheima.service;
import com.itheima.domain.User;
public interface UserService {
 public User queryUser(Integer id);
}

然后下面的紅色代碼就是解決上面的緩存穿透問題，使用鎖來解決緩存穿透問題，而且叫做雙重檢測鎖，為什么叫做雙重檢測鎖呢，因?yàn)橛袃蓚€if語句，第一個if語句就是為了減少走紅色代碼里面的同步代碼塊，因?yàn)槿绻麚Q成里面存在想要的數(shù)據(jù)，那么就不需要走下面的紅色代碼里面的同步代碼塊了，所以有兩個if語句，至于為什么要有下面的 user= userMap.get(id);，是因?yàn)榈谝淮尉€程查詢把數(shù)據(jù)放到模仿的redis緩存里面之后，剩下的線程當(dāng)走到下面的同步代碼塊的時候，需要在查詢一下緩存里面的數(shù)據(jù)就會發(fā)現(xiàn)剛剛第一個線程放到redis里面的數(shù)據(jù)了，所以才會有下面的紅色代碼里面的 user= userMap.get(id);

package com.itheima.service;
import com.itheima.domain.User;
import com.itheima.mapper.UserMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.HashMap;
import java.util.Map;
@Service
public class UserServiceImpl implements UserService {
 @Autowired
 private UserMapper userMapper;
 // 藍(lán)色代碼注釋開始
 static Map<Integer,User> userMap=new HashMap();
 static {
  User huancun_user =new User();
  huancun_user.setId(1);
  huancun_user.setName("zhangsan");
  huancun_user.setPassword("123");
  huancun_user.setName("張三");
  userMap.put(1,huancun_user);
 }
 // 藍(lán)色代碼注釋結(jié)束
 public User queryUser(Integer id){
  User user=userMap.get(id);
  // 紅色代碼注釋開始
  if(user==null){
   synchronized (this) {
    user= userMap.get(id);
    if (null == user) {
     user= userMapper.queryUser(id);
     System.out.println("查詢數(shù)據(jù)庫");
     userMap.put(user.getId(),user);
    }else{
     System.out.println("查詢緩存");
    }
   }
  }else{
   System.out.println("查詢緩存");
  }
  //紅色代碼注釋結(jié)束
  return user;
 };
}

數(shù)據(jù)庫里面的數(shù)據(jù)如下

-- ----------------------------
-- Table structure for `user`
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `username` varchar(50) DEFAULT NULL,
 `password` varchar(50) DEFAULT NULL,
 `name` varchar(50) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES ('1', 'zhangsan', '123', '張三');
INSERT INTO `user` VALUES ('2', 'lisi', '123', '李四');

然后打印結(jié)果如下，就是就只有第一個打印了查詢數(shù)據(jù)庫，然后剩下的都是查詢緩存了，這就是解決緩存穿透