Java SPI的简单小实例

 package com.wlf.service; public interface ITest { void saySomething(); }

 package com.wlf.service.impl; import com.wlf.service.ITest; public class ITestImpl1 implements ITest { @Override public void saySomething() { System.out.println("Hi, mia."); } }

 package com.wlf.service.impl; import com.wlf.service.ITest; public class ITestImpl2 implements ITest { @Override public void saySomething() { System.out.println("Hello, world."); } }

 package com.wlf.service; import java.util.Iterator; import java.util.ServiceLoader; public class TestServiceLoader { public static void main(String[] args) { ServiceLoader serviceLoader = ServiceLoader.load(ITest.class); Iterator iTests = serviceLoader.iterator(); while (iTests.hasNext()) { ITest iTest = iTests.next(); System.out.printf("loading %s\n", iTest.getClass().getName()); iTest.saySomething(); } } }

 /** * Lazily loads the available providers of this loader's service. * * 
 The iterator returned by this method first yields all of the * elements of the provider cache, in instantiation order. It then lazily * loads and instantiates any remaining providers, adding each one to the * cache in turn. * * 
 To achieve laziness the actual work of parsing the available * provider-configuration files and instantiating providers must be done by * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and * {@link java.util.Iterator#next next} methods can therefore throw a * {@link ServiceConfigurationError} if a provider-configuration file * violates the specified format, or if it names a provider class that * cannot be found and instantiated, or if the result of instantiating the * class is not assignable to the service type, or if any other kind of * exception or error is thrown as the next provider is located and * instantiated. To write robust code it is only necessary to catch {@link * ServiceConfigurationError} when using a service iterator. * * 
 If such an error is thrown then subsequent invocations of the * iterator will make a best effort to locate and instantiate the next * available provider, but in general such recovery cannot be guaranteed. * * 
Design Note * Throwing an error in these cases may seem extreme. The rationale for * this behavior is that a malformed provider-configuration file, like a * malformed class file, indicates a serious problem with the way the Java * virtual machine is configured or is being used. As such it is * preferable to throw an error rather than try to recover or, even worse, * fail silently.
 * * 
 The iterator returned by this method does not support removal. * Invoking its {@link java.util.Iterator#remove() remove} method will * cause an {@link UnsupportedOperationException} to be thrown. * * @implNote When adding providers to the cache, the {@link #iterator * Iterator} processes resources in the order that the {@link * java.lang.ClassLoader#getResources(java.lang.String) * ClassLoader.getResources(String)} method finds the service configuration * files. * * @return An iterator that lazily loads providers for this loader's *     service */ 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(); } }; }

 // Cached providers, in instantiation order private LinkedHashMap providers = new LinkedHashMap<>();

 // The current lazy-lookup iterator private LazyIterator lookupIterator;

 public boolean hasNext() { if (acc == null) { return hasNextService(); } else { PrivilegedAction action = new PrivilegedAction() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } } public S next() { if (acc == null) { return nextService(); } else { PrivilegedAction action = new PrivilegedAction() { public S run() { return nextService(); } }; return AccessController.doPrivileged(action, acc); } }

 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; } 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 } public boolean hasNext() { if (acc == null) { return hasNextService(); } else { PrivilegedAction action = new PrivilegedAction() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } }

 private static final String PREFIX = "META-INF/services/";

 pending = parse(service, configs.nextElement());