Mybatis为我们提供插件技术，在我们sql执行流程过程中创建SqlSession的四大对象进行自定义代码处理的分装，实现一些特殊的需求。
接口定义：
我们在mybatis中使用插件都要实现Interceptor接口：
public interface Interceptor {
//它将拦截对象的原有方法，因此他是插件的核心方法，可以通过invocation参数反射调用原来的方法
Object intercept(Invocation invocation) throws Throwable;
//target是被拦截的对象，作用是给拦截对象生成一个代理对象，并返回代理对象；
Object plugin(Object target);
//在plugin中配置所需要的参数，方法在初始化的时候被调用一次。
void setProperties(Properties properties);
}
插件的初始化：
插件的初始化是在Mybatis初始化的时候完成的，代码在XMLConfigBuilder中的pluginElement(XNode parent)：
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
//执行
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
//执行
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//执行
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
//执行
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
typeAliasesElement(root.evalNode("typeAliases"));
//解析插件
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
//插件的初始化
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();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
插件的使用：
从Sql执行流程中我们很容易明白插件的调用位置和代码：
executor = interceptorChain.pluginAll(executor);
public class InterceptorChain {
private final List interceptors = new ArrayList();
//调用plugs链
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
//可以根据Interceptor接口的定义返回的是代理对象
target = interceptor.plugin(target);
}
return target;
}
//添加plugs链
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
//获取plugs链
public List getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
这里的重点还是interceptor.plugin(target)创建代理的过程，这个创建代理的过程Mybatis给我们提供一个工具类org.apache.ibatis.plugin.Plugin可以为我们提供非常便利的创建代理对象：
public class Plugin implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private Map, Set> signatureMap;
//构造方法
private Plugin(Object target, Interceptor interceptor, Map, Set> signatureMap) {
this.target = target;//目标对象
this.interceptor = interceptor;//目标对象实现的接口
this.signatureMap = signatureMap;//Signature的注解信息
}
//静态方法获取代理
public static Object wrap(Object target, Interceptor interceptor) {
//解析plug实现类中的注解配置
Map, Set> signatureMap = getSignatureMap(interceptor);
//获取目标对象的Class对象
Class type = target.getClass();
//从signatureMap获取匹配的interfaces对象
Class[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
//创建代理
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}
//InvocationHandler 额外功能方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//signatureMap中获取匹配的方法
Set methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
//调用插件interceptor中的拦截方法
return interceptor.intercept(new Invocation(target, method, args));
}
//直接待用目标方法
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//解析plug实现类中的注解配置
private static Map, Set> getSignatureMap(Interceptor interceptor) {
//获取Interceptes注解
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
// plug实现类不存在Intercepts注解 则抛出异常
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
//获取配置的Singature数组
Signature[] sigs = interceptsAnnotation.value();
//容器用于保持目标对象和注解匹配成功信息
//key ：interceptor中要拦截的目标类（sqlSession的四大对象）
//value：要拦截的方法
Map, Set> signatureMap = new HashMap, Set>();
//便利注解中配置的Signature数组
for (Signature sig : sigs) {
Set methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet();
signatureMap.put(sig.type(), methods);
}
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, Set> signatureMap) {
Set> 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()]);
}
}
插件工具类注解案例：
//Intercepts 注解
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
Signature[] value();
}
//Signature注解
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
Class type();
String method();
Class[] args();
}
//案例
@Intercepts({
@Signature(type=Executor.class,
method= "query",
args = { MappedStatement.class, Object.class,RowBounds.class,
ResultHandler.class }),
@Signature(type = Executor.class,
method = "update",
args = { MappedStatement.class, Object.class }) })