MyBatis 插件逻辑
MyBatis 允许你在已映射语句执行过程中的某一点进行拦截调用。
默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：
Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
ParameterHandler (getParameterObject, setParameters)
ResultSetHandler (handleResultSets, handleOutputParameters)
StatementHandler (prepare, parameterize, batch, update, query)
只需实现 Interceptor 接口，并指定想要拦截的方法签名即可
import org.apache.ibatis.builder.StaticSqlSource;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlSource;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import java.lang.reflect.Field;
import java.util.Properties;
@Intercepts({
@Signature(
type = Executor.class,
method = "query",
args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}
)
})
public class MySqlPagingPlugin implements Interceptor {
private static final Integer MAPPED_STATEMENT_INDEX = 0;
private static final Integer PARAMETER_INDEX = 1;
private static final Integer ROW_BOUNDS_INDEX = 2;
@Override
public Object intercept(Invocation invocation) throws Throwable {
Object[] args = invocation.getArgs();
RowBounds rb = (RowBounds) args[ROW_BOUNDS_INDEX];
// 无需分页
if (rb == RowBounds.DEFAULT) {
return invocation.proceed();
}
// 将原 RowBounds 参数设为 RowBounds.DEFAULT，关闭 MyBatis 内置的分页机制
args[ROW_BOUNDS_INDEX] = RowBounds.DEFAULT;
MappedStatement ms = (MappedStatement) args[MAPPED_STATEMENT_INDEX];
BoundSql boundSql = ms.getBoundSql(args[PARAMETER_INDEX]);
// 获取 SQL 语句，拼接 limit 语句
String sql = boundSql.getSql();
String limit = String.format("LIMIT %d,%d", rb.getOffset(), rb.getLimit());
sql = sql + " " + limit;
// 创建一个 StaticSqlSource，并将拼接好的 sql 传入
SqlSource sqlSource = new StaticSqlSource(ms.getConfiguration(), sql, boundSql.getParameterMappings());
// 通过反射获取并设置 MappedStatement 的 sqlSource 字段
Field field = MappedStatement.class.getDeclaredField("sqlSource");
field.setAccessible(true);
field.set(ms, sqlSource);
// 执行被拦截方法
return invocation.proceed();
}
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
}
}
配置文件
<!-- mybatis-config.xml -->
<plugins>
<plugin interceptor="MySqlPagingPlugin" />
</plugins>
植入插件和执行插件的时序图：
public class XMLConfigBuilder extends BaseBuilder {
// 省略其他代码
private void pluginElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
// 将interceptor指定的名称解析为Interceptor类型
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
// 省略其他代码
}
添加插件实例到 interceptors 集合中
public class InterceptorChain {
// 省略其他代码
/** 添加插件实例到 interceptors 集合中 */
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
// 省略其他代码
}
植入插件
public class Configuration {
// 省略其他代码
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
// 根据 executorType 创建相应的 Executor 实例
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);
}
// 如果启用了缓存，则使用缓存执行器。
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
// 植入插件
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
// 省略其他代码
}
调用拦截器的 plugin 方法植入相应的插件逻辑
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public Object pluginAll(Object target) {
// 遍历拦截器集合
for (Interceptor interceptor : interceptors) {
// 调用拦截器的 plugin 方法植入相应的插件逻辑
target = interceptor.plugin(target);
}
return target;
}
// 省略其他代码
}
为目标对象生成代理
plugin 方法在内部调用了 Plugin 类的 wrap 方法，用于为目标对象生成代理。Plugin 类实现了 InvocationHandler 接口，因此它可以作为参数传给 Proxy 的 newProxyInstance 方法。
// MySqlPagingPlugin
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
为目标类生成代理类
// org/apache/ibatis/plugin/Plugin.java
public static Object wrap(Object target, Interceptor interceptor) {
/*
* 获取插件类 @Signature 注解内容，并生成相应的映射结构。形如下面：
* {
* Executor.class : [query, update, commit],
* ParameterHandler.class : [getParameterObject, setParameters]
* }
*/
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
// 获取目标类实现的接口
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
// 通过 JDK 动态代理为目标类生成代理类
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
获取Intercepts注解的value值
// org/apache/ibatis/plugin/Plugin.java
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
// 获取Intercepts注解
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
// issue #251
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
// 获取Intercepts注解的value值
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
// 获取 Signature 注解的 type 属性值
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
// 通过 Signature 注解中的 method 和 args 值构造Method实例
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;
}
获取目标类实现的接口
// org/apache/ibatis/plugin/Plugin.java
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
while (type != null) {
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
type = type.getSuperclass();
}
return interfaces.toArray(new Class<?>[interfaces.size()]);
}
Plugin 实现了 InvocationHandler 接口，因此它的 invoke 方法会拦截所有的方法调用。invoke 方法会对所拦截的方法进行检测，以决定是否执行插件逻辑
// org/apache/ibatis/plugin/Plugin.java
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
/*
* 获取被拦截方法列表，比如：
* signatureMap.get(Executor.class)，可能返回 [query, update, commit]
*/
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
// 检测方法列表是否包含被拦截的方法
if (methods != null && methods.contains(method)) {
// 执行插件逻辑
return interceptor.intercept(new Invocation(target, method, args));
}
// 执行被拦截的方法
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
执行插件逻辑
// MySqlPagingPlugin
public Object intercept(Invocation invocation) throws Throwable {
// 执行插件逻辑
// 执行被拦截方法
return invocation.proceed();
}
调用被拦截的方法
// org/apache/ibatis/plugin/Invocation.java
public Object proceed() throws InvocationTargetException, IllegalAccessException {
// 调用被拦截的方法
return method.invoke(target, args);
}
