常用技术之SPI ServiceLoader
时间：2022-05-11 11:22
ServiceLoader是jdk对SPI(Service Provider Interface)机制的实现, 让具体业务实现与接口分离，让接口可以自由扩展，是非常常用的技术
常用场景如下:
1. java nio java.nio.channels.spi.SelectorProvider
2. java.nio.charset.spi.CharsetProvider
3. java.nio.file.spi.FileSystemProvider
4. com.fasterxml.jackson.databind.Module
5. java.sql.Driver
源码解析
public static ServiceLoader load(Class service) {
//默认使用上下文类加载器加载
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static ServiceLoader load(Class service,
ClassLoader loader)
{
return new ServiceLoader<>(service, loader);
}
private ServiceLoader(Class svc, ClassLoader cl) {
service = Objects.requireNonNull(svc, "Service interface cannot be null");
loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
//清空上次的缓存，并生成一个懒迭代器
reload();
}
public void reload() {
providers.clear();
lookupIterator = new LazyIterator(service, loader);
}
//查找META-INF/services/+service 的文件，并解析
private boolean hasNextService() {
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;
}
//返回上次解析的到类名，并把类名放进providers变量中缓存
private S nextService() {
if (!hasNextService())
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 {
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
}
java.util.ServiceLoader#iterator迭代器设计模式，如果查询缓存providers中有数据，直接返回，否则交给LazyIterator查找
public Iterator iterator() {
return new Iterator() {
Iterator> 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();
}
};
}
