在Mybatis执行SQL的生命周期中,会使用插件进行埋点,主要包括Executor、StatementHandler、ParameterHandler和ResultSetHandler等。在执行到这些特殊节点时,就会触发拦截器的拦截方法。
通过自定义插件,我们可以对这些核心的节点中进行特殊处理,主要应用场景包括分页、记录日志、加解密等。
Mybatis插件的核心类包括:
Mybatis插件的工作流程如下:
创建Executor/StatementHandler/ParameterHandler/ResultSetHandler代理对象的方法都位于Configuration:
// org.apache.ibatis.session.Configuration#newExecutor
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
// 使用拦截器进行动态代理
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
// org.apache.ibatis.session.Configuration#newStatementHandler
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
// 使用拦截器进行动态代理
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
// org.apache.ibatis.session.Configuration#newParameterHandler
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
// 使用拦截器进行动态代理
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
// org.apache.ibatis.session.Configuration#newResultSetHandler
public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
ResultHandler resultHandler, BoundSql boundSql) {
ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
// 使用拦截器进行动态代理
resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
return resultSetHandler;
}
接下来我们来看创建Executor/StatementHandler/ParameterHandler/ResultSetHandler对象&进行代理的节点。
在使用SqlSessionFactory#openSession创建SqlSEession时,会创建Executor对象,并进行代理:
// org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSessionFromDataSource
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 创建Executor
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
在Executor执行SQL时,会创建StatementHandler,并进行代理:
// org.apache.ibatis.executor.SimpleExecutor#doQuery
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
// 创建StatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
ParameterHandler和ResultSetHandler作为StatementHandler的成员变量存在,会在其构造函数中进行创建和代理:
// BaseStatementHandler构造函数
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
// ……
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}
接下来我们来看InterceptorChain如何使用拦截器集合对Executor/StatementHandler/ParameterHandler/ResultSetHandler对象进行代理。
InterceptorChain会遍历拦截器集合,进行一层一层代理:
// org.apache.ibatis.plugin.InterceptorChain#pluginAll
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
每一次代理都会调用Plugin#wrap,它只是对JDK动态代理进行了简单应用:
// org.apache.ibatis.plugin.Interceptor#plugin
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}
// org.apache.ibatis.plugin.Plugin#wrap
public static Object wrap(Object target, Interceptor interceptor) {
// 获取拦截配置信息
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
Plugin本身实现了InvocationHandler方法,其中就定义了代理逻辑,主要会根据配置判断是否需要进行拦截,并执行对应方法:
// org.apache.ibatis.plugin.Plugin#invoke
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
// 从当前拦截器配置信息中获取当前方法的拦截信息
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
// 如果存在拦截配置,执行拦截器的拦截方法
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
// 如果不存在拦截配置,执行原始方法
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
上述拦截配置信息来自于Interceptor实现类上的@Intercepts和@Signature注解,通过Signature#type指定需要拦截的类,通过Signature#method和Signature#args共同指定需要拦截的方法:
// org.apache.ibatis.plugin.Plugin#getSignatureMap
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
// 获取@Intercepts注解
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
// 获取@Sinature注解
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
// 获取需要拦截的类(type)、方法(method和args)
for (Signature sig : sigs) {
Set<Method> methods = MapUtil.computeIfAbsent(signatureMap, sig.type(), k -> new HashSet<>());
try {
Method method = sig.type().getMethod(sig.method(), sig.args());
methods.add(method);
} catch (NoSuchMethodException e) {
throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
}
}
return signatureMap;
}
自定义拦截器主要有两个步骤:
创建拦截器只需要实现Interceptor接口:
public class CustomInterceptor implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
// 拦截业务处理
return null;
}
}
如果仅仅是进行切面处理(如记录日志),要记得执行代理对象的代理方法:
public Object intercept(Invocation invocation) throws Throwable {
// before……
// 获取代理信息
Object target = invocation.getTarget();
Method method = invocation.getMethod();
Object[] args = invocation.getArgs();
Object result = method.invoke(target, args);
// after……
return result;
}
如果需要执行自定义逻辑,甚至可以不执行代理对象的代理方法,完全由我们自己定义业务逻辑。
我们还需要指定需要拦截的类和方法,例如如果要拦截org.apache.ibatis.executor.Executor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)方法,我们可以添加如下注解:
@Intercepts(
{
@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})
}
)
我们还可以配置多个拦截类和方法,既可以是同一个类,也可以是不同类。
但是通常不推荐&不会为一个拦截器配置多个拦截类,因为这样会造成代码逻辑混乱,职责不明确。
创建拦截器很简单,但是最重要的是要选择适合的需要拦截的类和方法。
因为Executor/StatementHandler/ParameterHandler/ResultSetHandler的方法很多,在Mybatis执行SQL过程中,有些方法可能不会被触发。
这就对开发人员有两个要求:
注册拦截器,本质上是需要将自定义的拦截器添加到Mybatis的配置信息中(InterceptorChain)。
对于原生Mybatis或Mybatis-Spring场景中,可以直接使用Configuration#addInterceptor方法:
CustomInterceptor customInterceptor = new CustomInterceptor();
configuration.addInterceptor(customInterceptor);
如果使用Mybatis-SpringBoot框架,则只需要将拦截器注册为Bean添加到Spring容器中:
在自动配置过程中,会按以下流程注册拦截器:
读取容器中的Interceptor Bean对象
添加到SqlSessionFactoryBean
注册到Configuration
// org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration#MybatisAutoConfiguration
public MybatisAutoConfiguration(MybatisProperties properties, ObjectProvider> configurationCustomizersProvider, ObjectProvider
> sqlSessionFactoryBeanCustomizers) {
// 1、读取容器中的Interceptor Bean对象
this.interceptors = (Interceptor[])interceptorsProvider.getIfAvailable();
// ……
}
// org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration#sqlSessionFactory
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
if (!ObjectUtils.isEmpty(this.interceptors)) {
// 2、添加到SqlSessionFactoryBean
factory.setPlugins(this.interceptors);
}
// ……
}
protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
if (!isEmpty(this.plugins)) {
// 3、注册到Configuration
Stream.of(this.plugins).forEach(plugin -> {
targetConfiguration.addInterceptor(plugin);
});
}
}
基于Mybatis插件扩展的开源框架比较少,最常用、最热门的应该是PageHelper。
GitHub:https://github.com/pagehelper/Mybatis-PageHelper
它的原理是自定义了拦截器:com.github.pagehelper.PageInterceptor
在设置分页信息时,会将分页信息添加到线程变量中:
PageHelper.startPage(pageNum, pageSize);
// com.github.pagehelper.page.PageMethod#setLocalPage
protected static void setLocalPage(Page page) {
LOCAL_PAGE.set(page);
}
// com.github.pagehelper.page.PageMethod#LOCAL_PAGE
protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal<Page>();
在执行org.apache.ibatis.executor.Executor#query方法时,会触发该拦截器,如果线程变量中存在分页信息,进行分页逻辑。主要流程如下:
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