Mybatis源码解析之核心类分析
Mybatis源码解析之初始化分析
Mybatis源码解析之执行流程解析
Mybatis源码解析之数据库连接和连接池
Mybatis源码解析之事务管理
Mybatis源码解析之缓存机制(一):一级缓存
Mybatis源码解析之缓存机制(二):二级缓存
为了保证程序的扩展性,提供使用者在框架基础上增强自定义的逻辑处理,mybatis提供了一种基于拦截器的插件机制。
一、Interceptor接口
public interface Interceptor {
//覆盖原来的方法逻辑
Object intercept(Invocation invocation) throws Throwable;
//为被拦截的对象生成一个代理对象
Object plugin(Object target);
//设置配置的属性值
void setProperties(Properties properties);
}
在Mybatis中想实现插件就必须实现Interceptor接口。
二、InterceptorChain类
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
InterceptorChain:由Interceptor组成的列表类,pluginAll方法就是循环遍历Interceptor#plugin(Object)方法,将被拦截的对象层层进行代理,得到一个最终的代理对象。
InterceptorChain通过Configuration#pluginElement(XNode)记录配置的所有Interceptor实现类。
//Configuration#pluginElement(XNode)
private void pluginElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
三、 Intercepts和Signature注解
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
Signature[] value();
}
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
Class<?> type();
String method();
Class<?>[] args();
}
Intercepts注解用于Interceptor的实现类,用于注明拦截后需要处理的类和方法。
//如下,表示对Executor的两个query方法拦截后通过intercept方法覆盖原逻辑
@Intercepts(
{
@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),
}
)
四、拦截类
Mybatis支持Executor, StatementHandler, ParameterHandler, ResultSetHandler进行拦截,这是因为在Configuration类中生成对应对象时调用了InterceptorChain#pluginAll(Object)方法,返回的是代理对象。
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
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);
resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
return resultSetHandler;
}
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);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public Executor newExecutor(Transaction transaction) {
return newExecutor(transaction, defaultExecutorType);
}
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);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
五、Plugin类
Plugin继承了InvocationHandler接口,使用了jdk动态代理。
1. Plugin#wrap(Object, Interceptor)方法
public static Object wrap(Object target, Interceptor interceptor) {
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;
}
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
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());
}
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
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;
}
该方法是Mybatis提供的用于Interceptor#plugin(Object)生成代理对象的推荐方法。
可以看到,其通过Interceptor实现类上的Intercepts注解获取到需要拦截处理的类和其中的方法,组合成signatureMap。
2. Plugin#invoke(Object, Method, Object[])
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
可以看到,对于在signatureMap中的方法,用Interceptor#interceptor(Invocation)方法覆盖其原来逻辑。