动态代理概述
在学spring的时候知道使用动态代理实现aop,入门的列子：需要计算所有方法的调用时间。可以每个方法开始和结束都获取当前时间咋办呢。类似这样：
long current=system.currenttimemillis();
调用原来的方法
long last=system.currenttimemillis();
如果每个方法都人工加入实在有点不爽，动态代理出场了。动态代理利用字节码技在原来对应的类的子节码进行重写，添加对应的逻辑。
主流的动态代理实现技术主流如下：JDK 和CGLIB，Javassist，ASM 参考这个。
mybatis的应用
我们知道在mybatis中，我们只要编写mapper的接口和接口对应的mapper.xml ,利用下面方法就可以获取实例和调用方法。（代码来自mybatis3自带测试用例）
BoundBlogMapper mapper = session.getMapper(BoundBlogMapper.class); Blog b = mapper.selectBlogWithPostsUsingSubSelect(1);
assertEquals(1, b.getId());
session.close();
这里面的mapper就是利用jdk中的动态代理方式获取的接口BoundBlogMapper的代理类的对象。具体方法自己看。很容易跟踪。jdk动态代理最重要的就是实现
InvocationHandler接口的处理累。最重调用如下。invoke方法中mapperMethod.execute(sqlSession, args)最重要
public class MapperProxy<T> implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache;
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}
根据几种操作类型做不同处理
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} else {
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}