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尝试 pigeon 拦截器的时候，发现加载自定义拦截器的时候是通过 spi serviceloader 加载的，学习一波 spi。
某个接口，实现有多种，spi 能够为某个接口提供具体的实现类。先看下在 spi 机制下怎么在 pigeon 中自定义拦截器：
// 自定义了一个拦截器，实现了 pigeon 提供了拦截器接口
public class BusinessProviderInterceptor implements ProviderInterceptor {
@Override
public void preInvoke(ProviderContext providerContext) {LogTagContext.setLogTag("mobileno", "12345678");
}
@Override
public void postInvoke(ProviderContext providerContext) {}META-INF/services 下创立一个以接口命名的文件，文件里按行排列具体的实现类；文件里内容如下：
xxx.xxx.xxx.xxx.BusinessProviderInterceptor
#other selfinterceptor which implement ProviderInterceptor以上，能够在 pigeon 中实现自定义拦截器。
pigeon 是如何加载实例化这些拦截器的，具体如下：
//providerinterceptor 的执行在 BuinessProcessFilter 里，ProviderProcessInterceptorFactory 在废除，新的是 ProviderInterceptorFactory
List<ProviderProcessInterceptor> interceptors = ProviderProcessInterceptorFactory.getInterceptors();
for (ProviderProcessInterceptor interceptor : interceptors) {interceptor.preInvoke(request);
}
List<ProviderInterceptor> contextInterceptors = ProviderInterceptorFactory.getInterceptors();
for (ProviderInterceptor interceptor : contextInterceptors) {interceptorpublic class ProviderInterceptorFactory {
// 通过 ExtensionLoader 来加载 interceptor
private static List<ProviderInterceptor> interceptors = ExtensionLoader
.getExtensionList(ProviderInterceptor.class);
// 也能够手动调用这个办法来注册
public static boolean registerInterceptor(ProviderInterceptor providerContextInterceptor) {if (!interceptors.contains(providerContextInterceptor)) {return interceptors.add(providerContextInterceptor);
}
return false;
}
public static boolean unregisterInterceptor(ProviderInterceptor providerContextInterceptor) {return interceptors.remove(providerContextInterceptor);
}
public static List<ProviderInterceptor> getInterceptors() {return interceptors;}
}
public final class ExtensionLoader {
// 应用 map 来存储接口对应的实现，如果 map 中没有，则通过 serviceloader 来加载
private static Map<Class<?>, Object> extensionMap = new ConcurrentHashMap<Class<?>, Object>();
private static Map<Class<?>, List<?>> extensionListMap = new ConcurrentHashMap<Class<?>, List<?>>();
private ExtensionLoader() {}
public static <T> T getExtension(Class<T> clazz) {T extension = (T) extensionMap.get(clazz);
if (extension == null) {extension = newExtension(clazz);
if (extension != null) {extensionMap.put(clazz, extension);
}
}
return extension;
}
public static <T> List<T> getExtensionList(Class<T> clazz) {List<T> extensions = (List<T>) extensionListMap.get(clazz);
if (extensions == null) {extensions = newExtensionList(clazz);
if (!extensions.isEmpty()) {extensionListMap.put(clazz, extensions);
}
}
return extensions;
}
public static <T> T newExtension(Class<T> clazz) {
// here
ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
for (T service : serviceLoader) {return service;}
return null;
}
public static <T> List<T> newExtensionList(Class<T> clazz) {ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
List<T> extensions = new ArrayList<T>();
for (T service : serviceLoader) {extensions.add(service);
}
return extensions;
}
}
重点就是 ServiceLoader.load
// ServiceLoader 实现了 Iterable
public final class ServiceLoader<S>
implements Iterable<S>
{
// 目录门路
private static final String PREFIX = "META-INF/services/";
// 正在加载的类或接口
private Class<S> service;
// 用来实例化的 classloader
private ClassLoader loader;
// 按实例化的程序缓存实现类
private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
//ServiceLoader 自定义的一个外部类，次要实现了懒加载
private LazyIterator lookupIterator;
public void reload() {providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
service = svc;
loader = cl;
reload();}
private static void fail(Class service, String msg, Throwable cause)
throws ServiceConfigurationError
{throw new ServiceConfigurationError(service.getName() + ":" + msg,
cause);
}
private static void fail(Class service, String msg)
throws ServiceConfigurationError
{throw new ServiceConfigurationError(service.getName() + ":" + msg);
}
private static void fail(Class service, URL u, int line, String msg)
throws ServiceConfigurationError
{fail(service, u + ":" + line + ":" + msg);
}
// 解析文件，将文件中的 name 放到 list 中
private int parseLine(Class service, URL u, BufferedReader r, int lc,
List<String> names)
throws IOException, ServiceConfigurationError
{String ln = r.readLine();
if (ln == null) {return -1;}
int ci = ln.indexOf('#');
if (ci >= 0) ln = ln.substring(0, ci);
ln = ln.trim();
int n = ln.length();
if (n != 0) {if ((ln.indexOf('') >= 0) || (ln.indexOf('\t') >= 0))
fail(service, u, lc, "Illegal configuration-file syntax");
int cp = ln.codePointAt(0);
if (!Character.isJavaIdentifierStart(cp))
fail(service, u, lc, "Illegal provider-class name:" + ln);
for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {cp = ln.codePointAt(i);
if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
fail(service, u, lc, "Illegal provider-class name:" + ln);
}
// 加载过的不放在这个 list 里
if (!providers.containsKey(ln) && !names.contains(ln))
names.add(ln);
}
return lc + 1;
}
private Iterator<String> parse(Class service, URL u)
throws ServiceConfigurationError
{
InputStream in = null;
BufferedReader r = null;
ArrayList<String> names = new ArrayList<>();
try {in = u.openStream();
r = new BufferedReader(new InputStreamReader(in, "utf-8"));
int lc = 1;
while ((lc = parseLine(service, u, r, lc, names)) >= 0);
} catch (IOException x) {fail(service, "Error reading configuration file", x);
} finally {
try {if (r != null) r.close();
if (in != null) in.close();} catch (IOException y) {fail(service, "Error closing configuration file", y);
}
}
return names.iterator();}
// 懒加载的实现
private class LazyIterator
implements Iterator<S>
{
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;
Iterator<String> pending = null;
String nextName = null;
private LazyIterator(Class<S> service, ClassLoader loader) {
this.service = service;
this.loader = loader;
}
public boolean hasNext() {if (nextName != null) {return true;}
if (configs == null) {
try {String fullName = PREFIX + service.getName();
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {fail(service, "Error locating configuration files", x);
}
}
while ((pending == null) || !pending.hasNext()) {if (!configs.hasMoreElements()) {return false;}
pending = parse(service, configs.nextElement());
}
nextName = pending.next();
return true;
}
public S next() {if (!hasNext()) {throw new NoSuchElementException();
}
String cn = nextName;
nextName = null;
Class<?> c = null;
try {c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
fail(service,
"Provider" + cn + "not found");
}
if (!service.isAssignableFrom(c)) {
fail(service,
"Provider" + cn + "not a subtype");
}
try {
// 在 next 到某个实现类时，才真正的加载实例
S p = service.cast(c.newInstance());
providers.put(cn, p);
return p;
} catch (Throwable x) {
fail(service,
"Provider" + cn + "could not be instantiated",
x);
}
throw new Error(); // This cannot happen}
public void remove() {throw new UnsupportedOperationException();
}
}
public Iterator<S> iterator() {return new Iterator<S>() {
Iterator<Map.Entry<String,S>> knownProviders
= providers.entrySet().iterator();
public boolean hasNext() {if (knownProviders.hasNext())
return true;
return lookupIterator.hasNext();}
public S next() {if (knownProviders.hasNext())
return knownProviders.next().getValue();
return lookupIterator.next();}
public void remove() {throw new UnsupportedOperationException();
}
};
}
public static <S> ServiceLoader<S> load(Class<S> service,
ClassLoader loader)
{return new ServiceLoader<>(service, loader);
}
public static <S> ServiceLoader<S> load(Class<S> service) {ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {ClassLoader cl = ClassLoader.getSystemClassLoader();
ClassLoader prev = null;
while (cl != null) {
prev = cl;
cl = cl.getParent();}
return ServiceLoader.load(service, prev);
}
}
正文完
