wen mybatis 拦截器
1.mybatis拦截器介绍
拦截器可在mybatis进行sql底层处理的时候执行额外的逻辑，最常见的就是分页逻辑、对结果集进行处理过滤敏感信息等。
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;
}从上面的代码可以看到mybatis支持的拦截类型只有四种（按拦截顺序）
1.Executor 执行器接口
2.StatementHandler sql构建处理器
3.ParameterHandler 参数处理器
4.ResultSetHandler 结果集处理器
2.拦截器原理
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<>();
// 遍历定义的拦截器，对拦截的对象进行包装
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);
}
}
#Interceptor
public interface Interceptor {
Object intercept(Invocation invocation) throws Throwable;
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}
default void setProperties(Properties properties) {
// NOP
}
}mybatis拦截器本质上使用了jdk动态代理，interceptorChain拦截器链中存储了用户定义的拦截器，会遍历进行对目标对象代理包装。
用户自定义拦截器类需要实现Interceptor接口，以及实现intercept方法，plugin和setProperties方法可重写，plugin方法一般不会改动，该方法调用了Plugin的静态方法wrap实现了对目标对象的代理
public class Plugin implements InvocationHandler {
// 拦截目标对象
private final Object target;
// 拦截器对象-执行逻辑
private final Interceptor interceptor;
// 拦截接口和拦截方法的映射
private final Map<Class<?>, Set<Method>> signatureMap;
private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
this.target = target;
this.interceptor = interceptor;
this.signatureMap = signatureMap;
}
// 获取jdk代理对象
public static Object wrap(Object target, Interceptor interceptor) {
// 存储拦截接口和拦截方法的映射
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
// 获取拦截目标对象实现的接口，若为空则不代理
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
// 获取需要拦截的方法集合，若不存在则使用目标对象执行
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
// Invocation存储了目标对象、拦截方法以及方法参数
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
// 获取Intercepts注解值不能为空
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();
// key 拦截的类型
Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
try {
// 获取拦截的方法
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;
}
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<>();
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()]);
}
}@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
/**
* Returns method signatures to intercept.
*
* @return method signatures
*/
Signature[] value();
}
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
/**
* Returns the java type.
*
* @return the java type
*/
Class<?> type();
/**
* Returns the method name.
*
* @return the method name
*/
String method();
/**
* Returns java types for method argument.
* @return java types for method argument
*/
Class<?>[] args();
}可以看到，当被拦截的方法被执行时主要调用自定义拦截器的intercept方法，把拦截对象、方法以及方法参数封装成Invocation对象传递过去。
在getSignatureMap方法中可以看到，自定义的拦截器类上需要添加Intercepts注解并且Signature需要有值，Signature注解中的type为需要拦截对象的接口（Executor.class/StatementHandler/ParameterHandler/ResultSetHandler），method为需要拦截的方法的方法名，args为拦截方法的方法参数类型。
3.参考例子
接下来举一个拦截器实现对结果集下划线转驼峰的例子来简要说明
/**
* @author dxu2
* @date 2022/7/14
* map结果转驼峰
*/
@Intercepts(value = {@Signature(type = ResultSetHandler.class, method = "handleResultSets", args = {Statement.class})})
public class MyInterceptor implements Interceptor {
@SuppressWarnings("unchecked")
@Override
public Object intercept(Invocation invocation) throws Throwable {
// 调用目标方法
List<Object> result = (List<Object>) invocation.proceed();
for (Object o : result) {
if (o instanceof Map) {
processMap((Map<String, Object>) o);
} else {
break;
}
}
return result;
}
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
}
private void processMap(Map<String, Object> map) {
Set<String> keySet = new HashSet<>(map.keySet());
for (String key : keySet) {
if ((key.charAt(0) >= "A" && key.charAt(0) <= "Z") || key.indexOf("_") > 0) {
Object value = map.get(key);
map.remove(key);
map.put(camel(key), value);
}
}
}
// 下划线转驼峰
private String camel(String fieldName) {
StringBuffer stringBuffer = new StringBuffer();
boolean flag = false;
for (int i = 0; i < fieldName.length(); i++) {
if (fieldName.charAt(i) == "_") {
if (stringBuffer.length() > 0) {
flag = true;
}
} else {
if (flag) {
stringBuffer.append(Character.toUpperCase(fieldName.charAt(i)));
flag = false;
} else {
stringBuffer.append(Character.toLowerCase(fieldName.charAt(i)));
}
}
}
return stringBuffer.toString();
}
}这个例子拦截的是ResultSetHandler的handleResultSets方法，这个方法是用来对结果集处理的，看intercept方法首先调用了目标对象的方法接着强转为List<Object>类型，这里为什么可以强转呢，因为我们可以看到handleResultSets方法定义<E> List<E> handleResultSets(Statement stmt) throws SQLException; 返回的是List类型，然后遍历列表，若元素是map类型的再进行处理把key值转化为驼峰形式重新put到map中。
最后不要忘了把自定义的拦截器添加到配置中，这边是使用xml配置的，添加完后接着运行测试代码，可以看到列user_id已经转换成驼峰形式了。
<plugins>
<plugin interceptor="org.apache.ibatis.study.interceptor.MyInterceptor">
</plugin>
</plugins>#mapper接口
List<Map> selectAllUsers();
#mapper.xml
<select id="selectAllUsers" resultType="map">
select user_id, username, password, nickname
from user
</select>
#java测试类
public class Test {
public static void main(String[] args) throws IOException {
try (InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml")) {
// 构建session工厂 DefaultSqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
System.out.println(userMapper.selectAllUsers());
}
}
} 
