所有的框架都支持插件，我们可以用插件编写来扩展我们自己需要的功能，mybatis也不例外。因此，本文将从插件配置、插件编写、插件运行原理，插件注册与执行时机、初始化插件、分页插件原理来梳理说明。
插件配置
mybatis的分页插件配置在mybatis的构造器configuration，我们在初始化mybatis的时候就会独去这些插件，并且放到mybatis的拦截链InterceptorChain中，如：
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);
}
}
插件编写
我们自己写的插件必须要实现mybatis的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。
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 {
// 从 Invocation 中获取参数 final Object[] queryArgs = invocation.getArgs();
final MappedStatement ms = (MappedStatement) queryArgs[MAPPEDSTATEMENT_INDEX];
final Object parameter = queryArgs[PARAMETER_INDEX];
// 获取分页参数 Page paingParam = PageUtil.getPaingParam();
if (paingParam != null) {
// 构造新的 sql, select xxx from xxx where yyy limit offset,limit final BoundSql boundSql = ms.getBoundSql(parameter);
String pagingSql = getPagingSql(boundSql.getSql(), paingParam.getOffset(), paingParam.getLimit());
// 设置新的 MappedStatement BoundSql newBoundSql = new BoundSql(ms.getConfiguration(), pagingSql,
boundSql.getParameterMappings(), boundSql.getParameterObject());
MappedStatement mappedStatement = newMappedStatement(ms, newBoundSql);
queryArgs[MAPPEDSTATEMENT_INDEX] = mappedStatement;
// 重置 RowBound queryArgs[ROWBOUNDS_INDEX] = new RowBounds(RowBounds.NO_ROW_OFFSET, RowBounds.NO_ROW_LIMIT);
}
Object result = invocation.proceed();
PageUtil.removePagingParam();
return result;
}
@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"));
}
}
注解的作用：
@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。
如何执行这个插件呢？
请看自定义代码中的Plugin.wrap(target, this)，这是用JDK的动态代理机制来实现的，以此来实现方法的拦截和增强功能，执行后会回调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
mybatis可以拦截哪些对象呢？
请看mybatis的Configuration构造类
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。
interceptorChain.pluginAll()：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。
Invocation
调用执行拦截
public class Invocation {
private Object target;
private Method method;
private Object[] args;
}
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);
}
}
}
可以看到，上面并没有明确标明哪种拦截器，统一获取的是plugins的配置，即所有拦截器。因此对于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。
分页插件原理
我们通常用mybatis开发，都是手写sql，通过逻辑进行分页，那么不希望通过逻辑分页，传入sql直接分页，物理强行进行分页可否？这里就用到插件功能了。
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。