大多數框架，都支持插件，用戶可通過撰寫插件來自行擴展功能，Mybatis也不例外，
我們從插件配置、插件撰寫、插件運行原理、插件注冊與執行攔截的時機、初始化插件、分頁插件的原理等六個方面展開闡述，
1. 插件配置
Mybatis的插件配置在configuration內部，初始化時，會讀取這些插件，保存于Configuration物件的InterceptorChain中，整理了一份272頁Mybatis學習筆記
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE 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="https://www.cnblogs.com/bainannan/p/100" />
</plugin>
</plugins>
</configuration>
public class Configuration {
protected final InterceptorChain interceptorChain = new InterceptorChain();
}
org.apache.ibatis.plugin.InterceptorChain.java原始碼，
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
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<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
上面的for回圈代表了只要是插件，都會以責任鏈的方式逐一執行（別指望它能跳過某個節點），所謂插件，其實就類似于攔截器，
2. 如何撰寫一個插件
插件必須實作org.apache.ibatis.plugin.Interceptor介面，
public interface Interceptor {
Object intercept(Invocation invocation) throws Throwable;
Object plugin(Object target);
void setProperties(Properties properties);
}
intercept()方法：執行攔截內容的地方，比如想收點保護費，由plugin()方法觸發，interceptor.plugin(target)足以證明，
plugin()方法：決定是否觸發intercept()方法，
setProperties()方法：給自定義的攔截器傳遞xml配置的屬性引數，
下面自定義一個攔截器：
@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創建一個JDK的動態代理物件，觸發intercept()方法
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
value = https://www.cnblogs.com/bainannan/p/Integer.valueOf((String) properties.get("value"));
}
}
面對上面的代碼，我們需要解決兩個疑問：
1.為什么要寫Annotation注解？注解都是什么含義？
答： Mybatis規定插件必須撰寫Annotation注解，是必須，而不是可選，
@Intercepts注解：裝載一個@Signature串列，一個@Signature其實就是一個需要攔截的方法封裝，那么，一個攔截器要攔截多個方法，自然就是一個@Signature串列，
type = Executor.class,
method = "query",
args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }
解釋： 要攔截Executor介面內的query()方法，引數型別為args串列，
2. Plugin.wrap(target, this)是干什么的？
答： 使用JDK的動態代理，給target物件創建一個delegate代理物件，以此來實作方法攔截和增強功能，它會回呼intercept()方法，
org.apache.ibatis.plugin.Plugin.java原始碼：
public class Plugin implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private 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) {
// 創建JDK動態代理物件
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());
// 判斷是否是需要攔截的方法(很重要)
if (methods != null && methods.contains(method)) {
// 回呼intercept()方法
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//...
}
Map<Class<?>, Set> signatureMap：快取需攔截物件的反射結果，避免多次反射，即target的反射結果，
所以，我們不要動不動就說反射性能很差，那是因為你沒有像Mybatis一樣去快取一個物件的反射結果，
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是怎么判斷是否執行攔截內容的，要記住，
3. Mybatis可以攔截哪些介面物件？
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個介面物件內的方法，
重新審視interceptorChain.pluginAll()方法：該方法在創建上述4個介面物件時呼叫，其含義為給這些介面物件注冊攔截器功能，注意是注冊，而不是執行攔截，
攔截器執行時機：plugin()方法注冊攔截器后，那么，在執行上述4個介面物件內的具體方法時，就會自動觸發攔截器的執行，也就是插件的執行，
所以，一定要分清，何時注冊，何時執行，切不可認為pluginAll()或plugin()就是執行，它只是注冊，
4. Invocation
public class Invocation {
private Object target;
private Method method;
private Object[] args;
}
intercept(Invocation invocation)方法的引數Invocation ，我相信你一定可以看得懂，不解釋，
5. 初始化插件原始碼決議
org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XNode)方法部分原始碼，
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()方法的呼叫時機
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
對于Mybatis，它并不區分是何種攔截器介面，所有的插件都是Interceptor，Mybatis完全依靠Annotation去標識對誰進行攔截，所以，具備介面一致性，
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，市場上就出現了可以實作物理分頁的Mybatis的分頁插件，
要實作物理分頁，就需要對String sql進行攔截并增強，Mybatis通過BoundSql物件存盤String sql，而BoundSql則由StatementHandler物件獲取，整理了一份272頁Mybatis學習筆記
public interface StatementHandler {
<E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException;
BoundSql getBoundSql();
}
public class BoundSql {
public String getSql() {
return sql;
}
}
因此，就需要撰寫一個針對StatementHandler的query方法攔截器，然后獲取到sql，對sql進行重寫增強，
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標籤：Java
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