写过Mybatis插件?那说说自定义插件是如何加载的吧?
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2021-01-09 18:45
大多数框架,都支持插件,用户可通过编写插件来自行扩展功能,Mybatis也不例外。
我们从插件配置、插件编写、插件运行原理、插件注册与执行拦截的时机、初始化插件、分页插件的原理等六个方面展开阐述。
1. 插件配置
Mybatis的插件配置在configuration内部,初始化时,会读取这些插件,保存于Configuration对象的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源码。
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循环代表了只要是插件,都会以责任链的方式逐一执行(别指望它能跳过某个节点),所谓插件,其实就类似于拦截器。
2. 如何编写一个插件
插件必须实现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:执行拦截内容的地方,比如想收点保护费。由plugin()方法触发,interceptor.plugin(target)足以证明。
plugin()方法:决定是否触发intercept()方法。
setProperties()方法:给自定义的拦截器传递xml配置的属性参数。
下面自定义一个拦截器:
@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类是插件的核心类,用于给target创建一个JDK的动态代理对象,触发intercept()方法
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
value = Integer.valueOf((String) properties.get("value"));
}
}
面对上面的代码,我们需要解决两个疑问:
为什么要写Annotation注解?注解都是什么含义?
答:Mybatis规定插件必须编写Annotation注解,是必须,而不是可选。
@Intercepts注解:装载一个@Signature列表,一个@Signature其实就是一个需要拦截的方法封装。那么,一个拦截器要拦截多个方法,自然就是一个@Signature列表。
type = Executor.class, method = "query", args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }
解释:要拦截Executor接口内的query()方法,参数类型为args列表。
Plugin.wrap(target, this)是干什么的?
答:使用JDK的动态代理,给target对象创建一个delegate代理对象,以此来实现方法拦截和增强功能,它会回调intercept()方法。
org.apache.ibatis.plugin.Plugin.java源码:
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) {
// 创建JDK动态代理对象
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());
// 判断是否是需要拦截的方法(很重要)
if (methods != null && methods.contains(method)) {
// 回调intercept()方法
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//...
}
Map
所以,我们不要动不动就说反射性能很差,那是因为你没有像Mybatis一样去缓存一个对象的反射结果。
判断是否是需要拦截的方法,这句注释很重要,一旦忽略了,都不知道Mybatis是怎么判断是否执行拦截内容的,要记住。
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3. Mybatis可以拦截哪些接口对象?
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只能拦截ParameterHandler、ResultSetHandler、StatementHandler、Executor共4个接口对象内的方法。
重新审视interceptorChain.pluginAll()方法:该方法在创建上述4个接口对象时调用,其含义为给这些接口对象注册拦截器功能,注意是注册,而不是执行拦截。
拦截器执行时机:plugin()方法注册拦截器后,那么,在执行上述4个接口对象内的具体方法时,就会自动触发拦截器的执行,也就是插件的执行。
所以,一定要分清,何时注册,何时执行。切不可认为pluginAll()或plugin()就是执行,它只是注册。
4. Invocation
public class Invocation {
private Object target;
private Method method;
private Object[] args;
}
intercept(Invocation invocation)方法的参数Invocation ,我相信你一定可以看得懂,不解释。
5. 初始化插件源码解析
org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XNode)方法部分源码。
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();
// 这里展示了setProperties()方法的调用时机
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
对于Mybatis,它并不区分是何种拦截器接口,所有的插件都是Interceptor,Mybatis完全依靠Annotation去标识对谁进行拦截,所以,具备接口一致性。
6. 分页插件原理
由于Mybatis采用的是逻辑分页,而非物理分页,那么,市场上就出现了可以实现物理分页的Mybatis的分页插件。
要实现物理分页,就需要对String sql进行拦截并增强,Mybatis通过BoundSql对象存储String sql,而BoundSql则由StatementHandler对象获取。
public interface StatementHandler {
List query(Statement statement, ResultHandler resultHandler) throws SQLException ;
BoundSql getBoundSql();
}
public class BoundSql {
public String getSql() {
return sql;
}
}
因此,就需要编写一个针对StatementHandler的query方法拦截器,然后获取到sql,对sql进行重写增强。
任它天高海阔,任它变化无穷,我们只要懂得原理,再多插件,我们都可以对其投送王之蔑视。
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