1.Mybatis-Plugin的设计思路
听起来一个挺神奇的单词,插件。说白了就是使用了Jdk自带的动态代理.在需要的时候进行代理。AOP怎么用，他就怎么用。
Plugin类等价于InvocationHandler.Mybatis在调用Jdk代码如下:
Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
Plugin类继承InvocationHandler接口
public class Plugin implements InvocationHandler {
private final Object target;
private final Interceptor interceptor;
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;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
invoke方法执行的时候,也就是intercept被调用的时候.
用一句成语来讲，借鸡下蛋。借尸还魂(这个词还有点吓人)
2.写一个例子.使用Intecept接口,但不应用在Mybatis框架内。
思路:① 写一个接口+一个类②写一个Inteceptor对该接口进行增强。
--------------------接口----------------------------------
package org.mybatis.example.plugin;
public interface HelloWorld {
void sayHello(Integer v);
}
-------------------实现类---------------------------------
package org.mybatis.example.plugin;
public class HelloWorldImpl implements HelloWorld {
public void sayHello(Integer v){
System.out.println("Hello World!");
}
public static void main(String[] args) {
PluginUsedWithoutMyBatis plugin = new PluginUsedWithoutMyBatis();
HelloWorld helloWorld = (HelloWorld) plugin.plugin(new HelloWorldImpl());
helloWorld.sayHello(1);
}
}
--------------------Interceptor-----------------------------------
@Intercepts(@Signature(type= HelloWorld.class,method="sayHello",args={Integer.class}))
public class PluginUsedWithoutMyBatis implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
String methodName =invocation.getMethod().getName();
Object result =null;
if(methodName.equals("sayHello")){
System.out.println("method Begin:");
result =invocation.proceed();
System.out.println("method Begin:");
}else{
result =invocation.proceed();
}
return result;
}
@Override
public Object plugin(Object target) {
return Plugin.wrap(target,this);
}
@Override
public void setProperties(Properties properties) {
}
public static void main(String[] args) {
}
}
执行HelloWorldImpl中的Main方法，得到一段朴素的代码增强结果。
method Begin:
Hello World!
method Begin:
PluginUsedWithoutMyBatis类中需要注意的：
@Intercepts、@Signature这两个注解，限定了可以增强的接口，以及方法名称以及参数。
理解成过滤器。在Mybatis的Plugins配置中，所有的Plugin都配置在一起。根据Signature决定Interceptor对谁增强。
问题:
如果两个Interceptor都对同一个对象增强,结果如何?
写了另外一个代理类，对HelloWorldImpl进行二次增强，结果如下:
enhance Second Begin:
enhance First Begin:
Hello World!
enhance First end:
enhance Second end:
可以看到两次增强的结果。
3、回到Mybatis,是否可以对所有的MyBatis接口增强？
Mybatis的Configruation类，只在几个对象的创建时做了增强
StatementHandler 、ResultSetHandler 、ParameterHandler 、Executor
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 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 ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
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;
}
4、现有增强插件的实现:
Executor的query、update时候可以监控sql执行时间。监控慢SQL
Executor的query、update catch异常，当发生异常时，可以打印sql以及执行参数。以及具体错误消息。