spring+mybatis利用interceptor(plugin)兑现数据库读写分离

spring+mybatis利用interceptor(plugin)实现数据库读写分离

1. 前提

好长时间不写博客了，应该吐槽，写点什么东西了！最近在研究数据库读写分离，分表分库的一些东西。其实这个问题好早之前就想好，只是以前使用hibernate，难点是不好判断什么样的sql走读库，什么样的sql走主库？用正则匹配开头或许可以，/^select 没想出什么好的解决方法，mybatis就不一样了，mappedstatement有commandtype属性，象select，update，delete等类型，为实现读写分离打下来良好的基础。

2. 解决方法

LazyConnectionProxy + RoutingDataSource + Plugin

在SqlSessionTemplate，创建DefaultSqlSession的时候，使用connection proxy的代理，这时并没有真正的获取connection，因为我们不知道是要取读还是写的数据源。待到StatementHandler的prepare()使用connection创建PreparedStatement的时候再根据mappedstatement的commandType去路由获取真实的connection。

RoutingDataSource支持一主一从，或者一主多从并采用round robin的方式简单负载均衡，预留接口路由和负载均衡策略可自定义。

不支持事务，适合auto commit为true的场景。表述能力不强，有兴趣的可以看看代码，欢迎拍砖！

http://code.google.com/p/rwmybatis/

1 楼 mineral 2012-05-23

运行单元测试用例ATest.testCount()方法，无法通过测试。报NPE。

org.apache.ibatis.exceptions.PersistenceException:
### Error opening session. Cause: java.lang.NullPointerException
### Cause: java.lang.NullPointerException
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:8)
at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromConnection(DefaultSqlSessionFactory.java:106)
at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession(DefaultSqlSessionFactory.java:60)
at org.mybatis.spring.SqlSessionUtils.getSqlSession(SqlSessionUtils.java:134)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:333)
at $Proxy9.selectOne(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:154)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:75)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:38)
at $Proxy10.count(Unknown Source)
at com.test.rwmybatis.ATest.testCount(ATest.java:21)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:82)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:240)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:180)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: java.lang.NullPointerException
at $Proxy12.getAutoCommit(Unknown Source)
at org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromConnection(DefaultSqlSessionFactory.java:93)
... 37 more

2 楼 xiaoZ5919 2012-05-23

mineral 写道

灰常感谢您的反馈，会错的代码在RWConnectionInvocationHandler获取autocommit的值时候，在svn的中版本有点问题
else if (method.getName().equals("getAutoCommit")) {
//                    if (this.autoCommit != null) {
//                        return this.autoCommit;
//                    }
                    return null;
                    // Else fetch actual Connection and check there,
                    // because we didn't have a default specified.
                }
如有没有connectionproxy设置autocommit的值为返回null而导致问题的产生，这里有一个方案，就是初始化的时候挨个测试没有数据源的conneciton的autocommit的值并记录下来
不过我没有这样做，而是设定一个defaultautocommit为true并以及commit到svn中。这个项目只是玩票的，仅当做一个思路或者方案

spring+mybatis利用interceptor(plugin)兑现数据库读写分离

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