封装MybatisUtils解决线程安全与事务问题-创新互联

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MybatisUtils，封装获取mybatis的sqlSession 的方法。

需要解决2个问题：

1.1、线程安全问题

需要为单例模式。一个线程中，get到的sqlSession是同一个。

1.2、事务安全问题

（我对事物的理解，就是将几个行为，捆绑在一起，组合成一个事务。

在真正执行事务之前，会先预演一遍。

预演时，只有当所有的行为都执行成功时，才会去执行这个事务。保证每一个步骤都执行成功。

当其中一个行为执行失败，则整个事务执行失败。就不会去真正的执行这个失败的事物，每一个步骤，都没有真正的去执行。

举例：

A 给B 转账100元。数据库一共有2个行为，行为1是A扣款100元。行为2是B的余额中增加100元。

我们必须保证这两个行为同时成功或者同时失败，如果只有其中一个行为成功，另一个行为失败，都会造成事故。

所以我们将A给B转账的这个情景，这2个行为组成一个事务。（线程1上的一个事务）

A与B转账的同时（线程1），C也在给D转账（线程2），C与D 的行为组成一个事务，A与B的行为组成一个事务。

每个线程需要有自己的单独事务，则每个线程需要有自己单独sqlSession。

1.3 我们想要的最终效果

每个线程上的sqlSession是单例的。

但是线程与线程之间，每个线程都有一个属于自己线程的sqlSession。

我们这里通过内部静态类与ThreadLocal实现的。

二、代码 2.1 工具类MyBatisUtils

MyBatisUtils

package utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;


public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;
    // ThreadLocal,每个线程，都使用各自的一个SqlSession
    private static ThreadLocalthreadLocal = new ThreadLocal<>();

    static {
        InputStream in = null;
        try {
            in = Resources.getResourceAsStream("mybatis-config.xml");
        } catch (IOException e){
            e.printStackTrace();
        }
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(in,"test");
    }


    public static SqlSession getSqlSession(){
        SqlSession sqlSession = threadLocal.get();
        if(sqlSession==null){
            sqlSession= sqlSessionFactory.openSession(false);
            threadLocal.set(sqlSession);
        }
        return sqlSession;


    }

    public static void commit(){
        SqlSession sqlSession = threadLocal.get();
        if (sqlSession != null){
            sqlSession.commit();
        }

    }

    public static void closeSqlSession(){
        SqlSession sqlSession = threadLocal.get();
        if (sqlSession != null){
            sqlSession.close();
            threadLocal.remove();
        }
    }
}

注意：

1、sqlSession= sqlSessionFactory.openSession(false);

false 是不自动进行commit的，需要我们手动的进行commit。（事务的需要，）

2、ThreadLocal实现了每个线程有一个属于自己的 sqlSession

2.2 方法调用

package main;

import com.google.common.collect.Lists;
import entity.MyStudent;
import mapper.MyStudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import utils.MyBatisUtils;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;

public class App3 {
    public static void main(String[] args) throws IOException {
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        // 得到MyStudent表的mapper
        MyStudentMapper myStudentMapper = sqlSession.getMapper(MyStudentMapper.class);
        // 调用mapper的方法
        Listall = myStudentMapper.findAll();
        System.out.println("all = " + all);
        MyBatisUtils.closeSqlSession();

    }
}

2.3 证明线程内是单例的，多个线程间是不单例的。

package main;

import entity.MyStudent;
import mapper.MyStudentMapper;
import org.apache.ibatis.session.SqlSession;
import utils.MyBatisUtils;

import java.io.IOException;
import java.util.List;

public class App4 {
    public static void main(String[] args) throws IOException {
        for (int i=0;i<3;i++){
            new Thread(){
                @Override
                public void run() {
                    SqlSession sqlSession = MyBatisUtils.getSqlSession();
                    // 得到MyStudent表的mapper
                    MyStudentMapper myStudentMapper = sqlSession.getMapper(MyStudentMapper.class);
                    // 调用mapper的方法
                    Listall = myStudentMapper.findAll();
                    System.out.println("all = " + all);
                    System.out.println(this.getId()+"sqlSession = " + sqlSession);
                    SqlSession sqlSession2 = MyBatisUtils.getSqlSession();
                    System.out.println(this.getId()+"sqlSession2 = " + sqlSession2);
                    MyBatisUtils.closeSqlSession();
                }
            }.start();
        }




    }
}

打印结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=61163:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_321.jdk/Contents/Home/lib/tools.jar:/Users/zhaohui/IdeaProjects/MyTest/YourBatiesStudy/target/classes:/Users/zhaohui/.m2/repository/org/mybatis/mybatis/3.5.6/mybatis-3.5.6.jar:/Users/zhaohui/.m2/repository/mysql/mysql-connector-java/8.0.20/mysql-connector-java-8.0.20.jar:/Users/zhaohui/.m2/repository/org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar:/Users/zhaohui/.m2/repository/com/google/guava/guava/30.1.1-jre/guava-30.1.1-jre.jar:/Users/zhaohui/.m2/repository/com/google/guava/failureaccess/1.0.1/failureaccess-1.0.1.jar:/Users/zhaohui/.m2/repository/com/google/guava/listenablefuture/9999.0-empty-to-avoid-conflict-with-guava/listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar:/Users/zhaohui/.m2/repository/com/google/code/findbugs/jsr305/3.0.2/jsr305-3.0.2.jar:/Users/zhaohui/.m2/repository/org/checkerframework/checker-qual/3.8.0/checker-qual-3.8.0.jar:/Users/zhaohui/.m2/repository/com/google/errorprone/error_prone_annotations/2.5.1/error_prone_annotations-2.5.1.jar:/Users/zhaohui/.m2/repository/com/google/j2objc/j2objc-annotations/1.3/j2objc-annotations-1.3.jar:/Users/zhaohui/.m2/repository/org/projectlombok/lombok/1.18.18/lombok-1.18.18.jar main.App4
all = [MyStudent(id=201215121, name=李勇, age=20), MyStudent(id=201215122, name=刘晨, age=19), MyStudent(id=201215123, name=王敏, age=18), MyStudent(id=201215127, name=张三, age=19), MyStudent(id=201215128, name=张三, age=19), MyStudent(id=201215129, name=张三, age=19), MyStudent(id=201215130, name=张三, age=19), MyStudent(id=201215131, name=张三, age=19), MyStudent(id=201215132, name=张三, age=19), MyStudent(id=201215133, name=张三, age=19), MyStudent(id=201215134, name=张三三, age=19), MyStudent(id=201215135, name=李思思, age=19), MyStudent(id=201215136, name=张三, age=19), MyStudent(id=201215139, name=张三, age=19), MyStudent(id=201215140, name=张三三, age=19), MyStudent(id=201215141, name=李思思, age=19), MyStudent(id=201215142, name=张三, age=19), MyStudent(id=201215143, name=张三三, age=19), MyStudent(id=201215144, name=李思思, age=19)]
13sqlSession = org.apache.ibatis.session.defaults.DefaultSqlSession@1e57f15
13sqlSession2 = org.apache.ibatis.session.defaults.DefaultSqlSession@1e57f15
all = [MyStudent(id=201215121, name=李勇, age=20), MyStudent(id=201215122, name=刘晨, age=19), MyStudent(id=201215123, name=王敏, age=18), MyStudent(id=201215127, name=张三, age=19), MyStudent(id=201215128, name=张三, age=19), MyStudent(id=201215129, name=张三, age=19), MyStudent(id=201215130, name=张三, age=19), MyStudent(id=201215131, name=张三, age=19), MyStudent(id=201215132, name=张三, age=19), MyStudent(id=201215133, name=张三, age=19), MyStudent(id=201215134, name=张三三, age=19), MyStudent(id=201215135, name=李思思, age=19), MyStudent(id=201215136, name=张三, age=19), MyStudent(id=201215139, name=张三, age=19), MyStudent(id=201215140, name=张三三, age=19), MyStudent(id=201215141, name=李思思, age=19), MyStudent(id=201215142, name=张三, age=19), MyStudent(id=201215143, name=张三三, age=19), MyStudent(id=201215144, name=李思思, age=19)]
11sqlSession = org.apache.ibatis.session.defaults.DefaultSqlSession@2f73d49b
11sqlSession2 = org.apache.ibatis.session.defaults.DefaultSqlSession@2f73d49b
all = [MyStudent(id=201215121, name=李勇, age=20), MyStudent(id=201215122, name=刘晨, age=19), MyStudent(id=201215123, name=王敏, age=18), MyStudent(id=201215127, name=张三, age=19), MyStudent(id=201215128, name=张三, age=19), MyStudent(id=201215129, name=张三, age=19), MyStudent(id=201215130, name=张三, age=19), MyStudent(id=201215131, name=张三, age=19), MyStudent(id=201215132, name=张三, age=19), MyStudent(id=201215133, name=张三, age=19), MyStudent(id=201215134, name=张三三, age=19), MyStudent(id=201215135, name=李思思, age=19), MyStudent(id=201215136, name=张三, age=19), MyStudent(id=201215139, name=张三, age=19), MyStudent(id=201215140, name=张三三, age=19), MyStudent(id=201215141, name=李思思, age=19), MyStudent(id=201215142, name=张三, age=19), MyStudent(id=201215143, name=张三三, age=19), MyStudent(id=201215144, name=李思思, age=19)]
12sqlSession = org.apache.ibatis.session.defaults.DefaultSqlSession@4596a997
12sqlSession2 = org.apache.ibatis.session.defaults.DefaultSqlSession@4596a997

Process finished with exit code 0

同一个线程内的多个 sqlSession 的物理地址是一样的，

多个线程间， sqlSession 的物理地址是不一样的。

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