Mapper 与动态代理
Mapper 与动态代理
所谓动态代理，通过拦截器方法回调，对目标 target 方法进行增强。
自定义 JDK 动态代理实现 Mapper
首先定义一个 POJO。
public class User {
private Integer id;
private String name;
private int age;
public User(Integer id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
// getter setter
}
再定义一个接口 UserMapper.java。
public interface UserMapper {
public User getUserById(Integer id);
}
自定义一个 InvocationHandler。
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class MapperProxy implements InvocationHandler {
@SuppressWarnings("unchecked")
public <T> T newInstance(Class<T> clz) {
return (T) Proxy.newProxyInstance(clz.getClassLoader(), new Class[] { clz }, this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
// 诸如hashCode()、toString()、equals()等方法，将target指向当前对象this
return method.invoke(this, args);
} catch (Throwable t) {
}
}
// 投鞭断流
return new User((Integer) args[0], "zhangsan", 18);
}
}
上面代码中的 target，在执行 Object.java 内的方法时，target 被指向了 this。
public static void main(String[] args) {
MapperProxy proxy = new MapperProxy();
UserMapper mapper = proxy.newInstance(UserMapper.class);
User user = mapper.getUserById(1001);
System.out.println("ID:" + user.getId());
System.out.println("Name:" + user.getName());
System.out.println("Age:" + user.getAge());
System.out.println(mapper.toString());
}
Mybatis 自动映射器 Mapper
public static void main(String[] args) {
SqlSession sqlSession = MybatisSqlSessionFactory.openSession();
try {
StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
List<Student> students = studentMapper.findAllStudents();
for (Student student : students) {
System.out.println(student);
}
} finally {
sqlSession.close();
}
}
public interface StudentMapper {
List<Student> findAllStudents();
Student findStudentById(Integer id);
void insertStudent(Student student);
}
org.apache.ibatis.binding.MapperProxy.java 部分源码。
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;
}
@Override
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);
}
// ...
org.apache.ibatis.binding.MapperProxyFactory.java 部分源码。
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}