Java-SPI机制详解 - ToB企服应用市场:ToB评测及商务社交产业平台

private static final String PREFIX = "META-INF/services/";
private ServiceLoader(Class<S> 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;
reload();
}
/**
*
* 在调用该方法之后，迭代器方法的后续调用将延迟地从头开始查找和实例化提供程序，就像新创建的加载程序所做的那样
*/
public void reload() {
providers.clear(); //清除此加载程序的提供程序缓存，以便重新加载所有提供程序。
lookupIterator = new LazyIterator(service, loader);
}
private class LazyIterator implements Iterator<S>{
Class<S> service;
ClassLoader loader;
Enumeration<URL> configs = null;
Iterator<String> pending = null;
String nextName = null;
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
//找到配置文件
String fullName = PREFIX + service.getName();
//加载配置文件中的内容
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;
}
//解析配置文件
pending = parse(service, configs.nextElement());
}
//获取配置文件中内容
nextName = pending.next();
return true;
}
}
/**
*
* 通过反射实例化配置文件中的具体实现类
*/
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
}

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public interface IService {
/**
* 获取价格
* @return
*/
String getPrice();
/**
* 获取规格信息
* @return
*/
String getSpecifications();
}

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public class GoodServiceImpl implements IService {
@Override
public String getPrice() {
return "2000.00元";
}
@Override
public String getSpecifications() {
return "200g/件";
}
}

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public class MedicalServiceImpl implements IService {
@Override
public String getPrice() {
return "3022.12元";
}
@Override
public String getSpecifications() {
return "30粒/盒";
}
}

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org.example.GoodServiceImpl
org.example.MedicalServiceImpl

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public class Main {
public static void main(String[] args) {
final ServiceLoader<IService> serviceLoader = ServiceLoader.load(IService.class);
serviceLoader.forEach(service -> {
System.out.println(service.getPrice() + "=" + service.getSpecifications());
});
}
}

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2000.00元=200g/件
3022.12元=30粒/盒

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