前言
上一篇文章我们聊了一下[聊聊如何实现一个支持键值对的SPI]。本期我们来聊聊如何实现一个带有拦截器功能的SPI
什么是拦截器
指在某个方法或字段被访问之前进行拦截，然后在之前或之后加入某些操作
什么是拦截器链
指将拦截器按一定的顺序联结成一条链。在访问被拦截的方法或字段时，拦截器链中的拦截器就会按其之前定义的顺序被调用
实现拦截器逻辑
本文实现思路核心：利用责任链+动态代理
1、定义拦截器接口
public interface Interceptor {
int HIGHEST_PRECEDENCE = Integer.MIN_VALUE;
int LOWEST_PRECEDENCE = Integer.MAX_VALUE;
Object intercept(Invocation invocation) throws Throwable;
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}
int getOrder();
}
2、自定义需要被拦截器拦截的类接口注解
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
Signature[] value();
}
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
Class<?> type();
String method();
Class<?>[] args();
}
3、通过jdk动态代理实现拦截器调用逻辑
public class Plugin implements InvocationHandler {
private final Object target;
private final Interceptor interceptor;
private final 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) {
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());
if (methods != null && methods.contains(method)) {
Invocation invocation = new Invocation(target, method, args);
return interceptor.intercept(invocation);
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtils.unwrapThrowable(e);
}
}
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
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<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> 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()]);
}
}
4、构造出拦截器链
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<>();
public Object pluginAll(Object target) {
if(CollectionUtil.isNotEmpty(interceptors)){
for (Interceptor interceptor : getInterceptors()) {
target = interceptor.plugin(target);
}
}
return target;
}
public InterceptorChain addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
return this;
}
public List<Interceptor> getInterceptors() {
List<Interceptor> interceptorsByOrder = interceptors.stream().sorted(Comparator.comparing(Interceptor::getOrder).reversed()).collect(Collectors.toList());
return Collections.unmodifiableList(interceptorsByOrder);
}
}
5、通过拦截器链与之前实现的SPI绑定
@Activate
@Slf4j
public class InterceptorExtensionFactory implements ExtensionFactory {
private InterceptorChain chain;
@Override
public <T> T getExtension(final String key, final Class<T> clazz) {
if(Objects.isNull(chain)){
log.warn("No InterceptorChain Is Config" );
return null;
}
if (clazz.isInterface() && clazz.isAnnotationPresent(SPI.class)) {
ExtensionLoader<T> extensionLoader = ExtensionLoader.getExtensionLoader(clazz);
if (!extensionLoader.getSupportedExtensions().isEmpty()) {
if(StringUtils.isBlank(key)){
return (T) chain.pluginAll(extensionLoader.getDefaultActivate());
}
return (T) chain.pluginAll(extensionLoader.getActivate(key));
}
}
return null;
}
public InterceptorChain getChain() {
return chain;
}
public InterceptorExtensionFactory setChain(InterceptorChain chain) {
this.chain = chain;
return this;
}
示例演示
1、定义拦截器并指明要拦截的类接口方法
@Intercepts(@Signature(type = SqlDialect.class,method = "dialect",args = {}))
public class MysqlDialectInterceptor implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
System.out.println("MysqlDialectInterceptor");
return invocation.proceed();
}
@Override
public int getOrder() {
return HIGHEST_PRECEDENCE;
}
@Override
public Object plugin(Object target) {
if(target.toString().startsWith(MysqlDialect.class.getName())){
return Plugin.wrap(target,this);
}
return target;
}
}
2、构造拦截器链并设置到spi工厂中
@Before
public void before(){
InterceptorChain chain = new InterceptorChain();
chain.addInterceptor(new DialectInterceptor()).addInterceptor(new MysqlDialectInterceptor()).addInterceptor(new OracleDialectInterceptor());
factory = (InterceptorExtensionFactory) (ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getActivate("interceptor"));
factory.setChain(chain);
}
3、测试
@Test
public void testMysqlDialectInterceptor(){
SqlDialect dialect = factory.getExtension("mysql",SqlDialect.class);
Assert.assertEquals("mysql",dialect.dialect());
}
从控制台输出就可以看出已经把拦截器调用逻辑实现出来
总结
看了本篇的拦截器实现，眼尖的朋友就会发现，你这不就是抄mybatis拦截器的实现。确实是这样，但我更愿意不要脸的称这个为学以致用。mybatis的拦截器实现确实挺巧妙的，因为我们常规实现拦截器链调用正常是使用类似递归的方式，mybatis却借助了动态代理。
当然本篇的拦截器也加了一点彩蛋，比如增加了原生mybatis拦截器没提供的自定义执行顺序功能，原生的mybatis拦截器只能拦截Executor、ParameterHandler 、StatementHandler 、ResultSetHandler。而本文则没这个限制，但有个注意点是因为拦截器实现是基于jdk动态代理，因此自定义注解的class只能指定为接口，而不能是具体实现