类似于Java 1.5中的ServiceLoader吗？

问题描述

如何在实现定义接口的类路径中发现运行时的类？

How do I discover classes at runtime in the classpath which implements a defined interface?

ServiceLoader非常适合（我想，我还没有使用它），但是我需要在Java 1.5中完成它。

ServiceLoader suits well (I think, I haven't used it), but I need do it in Java 1.5.

推荐答案

Java 1.5中没有内置任何内容。我自己实施了;这不是太复杂。但是，当我们升级到Java 6时，我将不得不通过调用 ServiceLoader 来替换对我的实现的调用。我可以在应用程序和加载器之间定义一个小桥梁，但我只在几个地方使用它，并且包装器本身将是ServiceLoader的一个很好的候选者。

There's nothing built into Java 1.5 for this. I implemented it myself; it's not too complicated. However, when we upgrade to Java 6, I will have to replace calls to my implementation with calls to ServiceLoader. I could have defined a little bridge between the app and the loader, but I only use it in a few places, and the wrapper itself would be a good candidate for a ServiceLoader.

这是核心理念：

public <S> Iterable<S> load(Class<S> ifc) throws Exception {
  ClassLoader ldr = Thread.currentThread().getContextClassLoader();
  Enumeration<URL> e = ldr.getResources("META-INF/services/" + ifc.getName());
  Collection<S> services = new ArrayList<S>();
  while (e.hasMoreElements()) {
    URL url = e.nextElement();
    InputStream is = url.openStream();
    try {
      BufferedReader r = new BufferedReader(new InputStreamReader(is, "UTF-8"));
      while (true) {
        String line = r.readLine();
        if (line == null)
          break;
        int comment = line.indexOf('#');
        if (comment >= 0)
          line = line.substring(0, comment);
        String name = line.trim();
        if (name.length() == 0)
          continue;
        Class<?> clz = Class.forName(name, true, ldr);
        Class<? extends S> impl = clz.asSubclass(ifc);
        Constructor<? extends S> ctor = impl.getConstructor();
        S svc = ctor.newInstance();
        services.add(svc);
      }
    }
    finally {
      is.close();
    }
  }
  return services;
}

更好的异常处理留给读者练习。此外，该方法可以参数化以接受调用者选择的ClassLoader。

Better exception handling is left as an exercise for the reader. Also, the method could be parameterized to accept a ClassLoader of the caller's choosing.

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