外掛
摘要：
原理
使用的是責任鏈模式,在四大物件排程的時候,使用外掛執行程式碼,去執行操作
即初始化的時候在Config物件中,由於是單例,全域性唯一,生成的外掛物件儲存進入陣列中,這個外掛需要實現一個介面,然後在使用外掛的時候,通過責任鏈的方式,迴圈呼叫,進行處理.
核心在於下...
原理
使用的是責任鏈模式,在四大物件排程的時候,使用外掛執行程式碼,去執行操作
即初始化的時候在Config物件中,由於是單例,全域性唯一,生成的外掛物件儲存進入陣列中,這個外掛需要實現一個介面,然後在使用外掛的時候,通過責任鏈的方式,迴圈呼叫,進行處理.
核心在於下面這一段
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
使用外掛
使用外掛需要實現介面Interceptor
package com.ming.MyBatis;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Invocation;
import java.util.Properties;
public class PluginElement implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
return null;
}
@Override
public Object plugin(Object target) {
return null;
}
@Override
public void setProperties(Properties properties) {
}
}
外掛代理和反射設計
首先進入執行器
executor = (Executor) interceptorChain.pluginAll(executor);
然後執行器執行在這
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
plugin為生成代理物件的方法,逐步傳遞.
一層一層攔截處理
MyBatis提供了外掛類,用來生成代理物件,用來上方迴圈
/**
*Copyright 2009-2019 the original author or authors.
*
*Licensed under the Apache License, Version 2.0 (the "License");
*you may not use this file except in compliance with the License.
*You may obtain a copy of the License at
*
*http://www.apache.org/licenses/LICENSE-2.0
*
*Unless required by applicable law or agreed to in writing, software
*distributed under the License is distributed on an "AS IS" BASIS,
*WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
*See the License for the specific language governing permissions and
*limitations under the License.
*/
package org.apache.ibatis.plugin;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import org.apache.ibatis.reflection.ExceptionUtil;
/**
* @author Clinton Begin
*/
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;
}
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;
}
@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);
}
}
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<>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
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;
}
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()]);
}
}
這是一個動態代理物件.
wrap是生成的動態代理物件.
invoke是排程的方法.
如果存在攔截簽名,將會在這裡呼叫
若不存在攔截簽名,將會直接呼叫原生的方法
外掛開發
看上方的程式碼,可以知道需要確定攔截簽名用來呼叫開發好的外掛
如果需要攔截StatementHandler 執行SQL的過程
需要先檢視StatementHandler介面
/**
*Copyright 2009-2016 the original author or authors.
*
*Licensed under the Apache License, Version 2.0 (the "License");
*you may not use this file except in compliance with the License.
*You may obtain a copy of the License at
*
*http://www.apache.org/licenses/LICENSE-2.0
*
*Unless required by applicable law or agreed to in writing, software
*distributed under the License is distributed on an "AS IS" BASIS,
*WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
*See the License for the specific language governing permissions and
*limitations under the License.
*/
package org.apache.ibatis.executor.statement;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.List;
import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.parameter.ParameterHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.session.ResultHandler;
/**
* @author Clinton Begin
*/
public interface StatementHandler {
Statement prepare(Connection connection, Integer transactionTimeout)
throws SQLException;
void parameterize(Statement statement)
throws SQLException;
void batch(Statement statement)
throws SQLException;
int update(Statement statement)
throws SQLException;
<E> List<E> query(Statement statement, ResultHandler resultHandler)
throws SQLException;
<E> Cursor<E> queryCursor(Statement statement)
throws SQLException;
BoundSql getBoundSql();
ParameterHandler getParameterHandler();
}
開發外掛需要實現Interceptor介面
通過看介面定義可以知道需要攔截
Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException;
方法
所以編寫註解,提供簽名如下
// 說明這是攔截器
@Intercepts({
// 需要攔截的方法
@Signature(
type = StatementHandler.class,
method = "prepare",
args = {
// 提供引數
Connection.class
}
)
})
下面需要實現該介面
package com.ming.MyBatis.plugin;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.*;
import java.util.Properties;
/**
* 對資料庫返回的資料進行限制外掛
* @author ming
*/
// 說明這是攔截器
@Intercepts({
// 需要攔截的方法
@Signature(
type = Executor.class,
method = "update",
args = {
// 提供引數
MappedStatement.class,
Object.class
}
)
})
public class MyPlugin implements Interceptor {
Properties properties = null;
/**
* 用於替換攔截方法的內容,在這裡,原始方法被攔截
* @param invocation此為處理該內容的責任鏈
* @return
* @throws Throwable
*/
@Override
public Object intercept(Invocation invocation) throws Throwable {
// 處理之前
System.out.println("before.....");
// 進行呼叫真實的物件,否則排程到下一個代理物件invoke方法
Object object = invocation.proceed();
// 處理之後
System.out.println("after....");
return object;
}
/**
* 生成物件代理,用於原始碼中的迴圈 即呼叫的是這一句 target = interceptor.plugin(target);
* 在這裡,targer為外掛物件,然後在這裡進行迴圈處理,使用預設的wrap進行呼叫攔截的方法
* @param target
* @return
*/
@Override
public Object plugin(Object target) {
System.out.println("呼叫生成代理物件");
// 使用的是wrap方法,用於生成代理物件
return Plugin.wrap(target, this);
}
/**
* 從配置檔案中,獲取配置屬性 初始化的時候會呼叫此方法
* @param properties
*/
@Override
public void setProperties(Properties properties) {
System.out.println(properties.get("dbType"));
this.properties = properties;
}
}
一個是用來呼叫真實方法的.一個是用來生成代理物件的時候迴圈生成.最後是讀取內容
書寫完成以後更改配置檔案
<plugins> <plugin interceptor="com.ming.MyBatis.plugin.MyPlugin"> <property name="dbType" value="mysql"/> </plugin> </plugins>
然後執行.
檢視日誌資訊,可以看到
[2019-04-21 01:16:41,506] Artifact mingmingxiao:war exploded: Artifact is deployed successfully [2019-04-21 01:16:41,506] Artifact mingmingxiao:war exploded: Deploy took 2,203 milliseconds mysql 呼叫生成代理物件 呼叫生成代理物件 呼叫生成代理物件 呼叫生成代理物件 呼叫生成代理物件 21-Apr-2019 01:16:49.028 資訊 [Catalina-utility-1] org.apache.catalina.startup.HostConfig.deployDirectory 把web 應用程式部署到目錄 [/home/ming/apache-tomcat-9.0.17/webapps/manager] 21-Apr-2019 01:16:49.107 資訊 [Catalina-utility-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/home/ming/apache-tomcat-9.0.17/webapps/manager] has finished in [79] ms 呼叫生成代理物件
在這裡,完成一個外掛的開發