寫過Mybatis插件？那說說自定義插件是如何加載的吧？

大多數(shù)框架，都支持插件，用戶可通過編寫插件來自行擴(kuò)展功能，Mybatis也不例外。

我們從插件配置、插件編寫、插件運(yùn)行原理、插件注冊(cè)與執(zhí)行攔截的時(shí)機(jī)、初始化插件、分頁插件的原理等六個(gè)方面展開闡述。

1. 插件配置

Mybatis的插件配置在configuration內(nèi)部，初始化時(shí)，會(huì)讀取這些插件，保存于Configuration對(duì)象的InterceptorChain中。

configuration?PUBLIC?"-//mybatis.org//DTD?Config?3.0//EN"?"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
????<plugins>
??<plugin?interceptor="com.mybatis3.interceptor.MyBatisInterceptor">
???<property?name="value"?value="100"?/>
??plugin>
?plugins>
configuration>
public?class?Configuration?{
????protected?final?InterceptorChain?interceptorChain?=?new?InterceptorChain();
}

org.apache.ibatis.plugin.InterceptorChain.java源碼。

public?class?InterceptorChain?{

??private?final?List?interceptors?=?new?ArrayList();

??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?getInterceptors()?{
????return?Collections.unmodifiableList(interceptors);
??}

}

上面的for循環(huán)代表了只要是插件，都會(huì)以責(zé)任鏈的方式逐一執(zhí)行（別指望它能跳過某個(gè)節(jié)點(diǎn)），所謂插件，其實(shí)就類似于攔截器。

2. 如何編寫一個(gè)插件

插件必須實(shí)現(xiàn)org.apache.ibatis.plugin.Interceptor接口。

public?interface?Interceptor?{
??
??Object?intercept(Invocation?invocation)?throws?Throwable;

??Object?plugin(Object?target);

??void?setProperties(Properties?properties);

}

intercept()方法：執(zhí)行攔截內(nèi)容的地方，比如想收點(diǎn)保護(hù)費(fèi)。由plugin()方法觸發(fā)，interceptor.plugin(target)足以證明。

plugin()方法：決定是否觸發(fā)intercept()方法。

setProperties()方法：給自定義的攔截器傳遞xml配置的屬性參數(shù)。

下面自定義一個(gè)攔截器：

@Intercepts({
??@Signature(type?=?Executor.class,?method?=?"query",?args?=?{?MappedStatement.class,?Object.class,
????RowBounds.class,?ResultHandler.class?}),
??@Signature(type?=?Executor.class,?method?=?"close",?args?=?{?boolean.class?})?})
public?class?MyBatisInterceptor?implements?Interceptor?{

?private?Integer?value;

?@Override
?public?Object?intercept(Invocation?invocation)?throws?Throwable?{
??return?invocation.proceed();
?}

?@Override
?public?Object?plugin(Object?target)?{
??System.out.println(value);
????????//?Plugin類是插件的核心類，用于給target創(chuàng)建一個(gè)JDK的動(dòng)態(tài)代理對(duì)象，觸發(fā)intercept()方法
??return?Plugin.wrap(target,?this);
?}

?@Override
?public?void?setProperties(Properties?properties)?{
??value?=?Integer.valueOf((String)?properties.get("value"));
?}

}

面對(duì)上面的代碼，我們需要解決兩個(gè)疑問：

1. 為什么要寫Annotation注解？注解都是什么含義？

答：Mybatis規(guī)定插件必須編寫Annotation注解，是必須，而不是可選。

@Intercepts注解：裝載一個(gè)@Signature列表，一個(gè)@Signature其實(shí)就是一個(gè)需要攔截的方法封裝。那么，一個(gè)攔截器要攔截多個(gè)方法，自然就是一個(gè)@Signature列表。

type = Executor.class, method = "query", args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }

解釋：要攔截Executor接口內(nèi)的query()方法，參數(shù)類型為args列表。

2. Plugin.wrap(target, this)是干什么的？

答：使用JDK的動(dòng)態(tài)代理，給target對(duì)象創(chuàng)建一個(gè)delegate代理對(duì)象，以此來實(shí)現(xiàn)方法攔截和增強(qiáng)功能，它會(huì)回調(diào)intercept()方法。

org.apache.ibatis.plugin.Plugin.java源碼：

public?class?Plugin?implements?InvocationHandler?{

??private?Object?target;
??private?Interceptor?interceptor;
??private?Map,?Set>?signatureMap;

??private?Plugin(Object?target,?Interceptor?interceptor,?Map,?Set>?signatureMap)?{
????this.target?=?target;
????this.interceptor?=?interceptor;
????this.signatureMap?=?signatureMap;
??}

??public?static?Object?wrap(Object?target,?Interceptor?interceptor)?{
????Map,?Set>?signatureMap?=?getSignatureMap(interceptor);
????Class?type?=?target.getClass();
????Class[]?interfaces?=?getAllInterfaces(type,?signatureMap);
????if?(interfaces.length?>?0)?{
??????//?創(chuàng)建JDK動(dòng)態(tài)代理對(duì)象
??????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?methods?=?signatureMap.get(method.getDeclaringClass());
??????//?判斷是否是需要攔截的方法(很重要)
??????if?(methods?!=?null?&&?methods.contains(method))?{
????????//?回調(diào)intercept()方法
????????return?interceptor.intercept(new?Invocation(target,?method,?args));
??????}
??????return?method.invoke(target,?args);
????}?catch?(Exception?e)?{
??????throw?ExceptionUtil.unwrapThrowable(e);
????}
??}
//...
}

Map, Set> signatureMap：緩存需攔截對(duì)象的反射結(jié)果，避免多次反射，即target的反射結(jié)果。

所以，我們不要?jiǎng)硬粍?dòng)就說反射性能很差，那是因?yàn)槟銢]有像Mybatis一樣去緩存一個(gè)對(duì)象的反射結(jié)果。

判斷是否是需要攔截的方法，這句注釋很重要，一旦忽略了，都不知道Mybatis是怎么判斷是否執(zhí)行攔截內(nèi)容的，要記住。

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3. Mybatis可以攔截哪些接口對(duì)象？

public?class?Configuration?{
//...
public?ParameterHandler?newParameterHandler(MappedStatement?mappedStatement,?Object?parameterObject,?BoundSql?boundSql)?{
????ParameterHandler?parameterHandler?=?mappedStatement.getLang().createParameterHandler(mappedStatement,?parameterObject,?boundSql);
????parameterHandler?=?(ParameterHandler)?interceptorChain.pluginAll(parameterHandler);?//?1
????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);?//?2
????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);?//?3
????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);?//?4
????return?executor;
??}
//...
}

Mybatis只能攔截ParameterHandler、ResultSetHandler、StatementHandler、Executor共4個(gè)接口對(duì)象內(nèi)的方法。

重新審視interceptorChain.pluginAll()方法：該方法在創(chuàng)建上述4個(gè)接口對(duì)象時(shí)調(diào)用，其含義為給這些接口對(duì)象注冊(cè)攔截器功能，注意是注冊(cè)，而不是執(zhí)行攔截。

攔截器執(zhí)行時(shí)機(jī)：plugin()方法注冊(cè)攔截器后，那么，在執(zhí)行上述4個(gè)接口對(duì)象內(nèi)的具體方法時(shí)，就會(huì)自動(dòng)觸發(fā)攔截器的執(zhí)行，也就是插件的執(zhí)行。

所以，一定要分清，何時(shí)注冊(cè)，何時(shí)執(zhí)行。切不可認(rèn)為pluginAll()或plugin()就是執(zhí)行，它只是注冊(cè)。

4. Invocation

public?class?Invocation?{
??private?Object?target;
??private?Method?method;
??private?Object[]?args;
}

intercept(Invocation invocation)方法的參數(shù)Invocation ，我相信你一定可以看得懂，不解釋。

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5. 初始化插件源碼解析

org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XNode)方法部分源碼。

pluginElement(root.evalNode("plugins"));

?private?void?pluginElement(XNode?parent)?throws?Exception?{
????if?(parent?!=?null)?{
??????for?(XNode?child?:?parent.getChildren())?{
????????String?interceptor?=?child.getStringAttribute("interceptor");
????????Properties?properties?=?child.getChildrenAsProperties();
????????Interceptor?interceptorInstance?=?(Interceptor)?resolveClass(interceptor).newInstance();
????????//?這里展示了setProperties()方法的調(diào)用時(shí)機(jī)
????????interceptorInstance.setProperties(properties);
????????configuration.addInterceptor(interceptorInstance);
??????}
????}
??}

對(duì)于Mybatis，它并不區(qū)分是何種攔截器接口，所有的插件都是Interceptor，Mybatis完全依靠Annotation去標(biāo)識(shí)對(duì)誰進(jìn)行攔截，所以，具備接口一致性。

6. 分頁插件原理

由于Mybatis采用的是邏輯分頁，而非物理分頁，那么，市場(chǎng)上就出現(xiàn)了可以實(shí)現(xiàn)物理分頁的Mybatis的分頁插件。

要實(shí)現(xiàn)物理分頁，就需要對(duì)String sql進(jìn)行攔截并增強(qiáng)，Mybatis通過BoundSql對(duì)象存儲(chǔ)String sql，而BoundSql則由StatementHandler對(duì)象獲取。

public?interface?StatementHandler?{
?????List?query(Statement?statement,?ResultHandler?resultHandler)?throws?SQLException;

????BoundSql?getBoundSql();
}
public?class?BoundSql?{
???public?String?getSql()?{
????return?sql;
??}
}

因此，就需要編寫一個(gè)針對(duì)StatementHandler的query方法攔截器，然后獲取到sql，對(duì)sql進(jìn)行重寫增強(qiáng)。

任它天高海闊，任它變化無窮，我們只要懂得原理，再多插件，我們都可以對(duì)其投送王之蔑視。

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