在Java中，是否可以知道某个类是否已被加载？ [英] In Java, is it possible to know whether a class has already been loaded?

问题描述

是否可以知道是否已加载Java类，没有尝试加载它？ Class.forName 尝试加载该类，但我不希望出现这种副作用。还有另外一种方法吗？

Is it possible to know whether a Java class has been loaded, without attempting to load it? Class.forName attempts to load the class, but I don't want this side effect. Is there another way?

（我不想覆盖类加载器。我正在寻找一种相对简单的方法。）

(I don't want to override the class loader. I'm looking for a relatively simple method.)

推荐答案

（感谢Aleksi）此代码：

(Thanks to Aleksi) This code:

public class TestLoaded {
     public static void main(String[] args) throws Exception {
          java.lang.reflect.Method m = ClassLoader.class.getDeclaredMethod("findLoadedClass", new Class[] { String.class });
          m.setAccessible(true);
          ClassLoader cl = ClassLoader.getSystemClassLoader();
          Object test1 = m.invoke(cl, "TestLoaded$ClassToTest");
          System.out.println(test1 != null);
          ClassToTest.reportLoaded();
          Object test2 = m.invoke(cl, "TestLoaded$ClassToTest");
          System.out.println(test2 != null);
     }
     static class ClassToTest {
          static {
               System.out.println("Loading " + ClassToTest.class.getName());
          }
          static void reportLoaded() {
               System.out.println("Loaded");
          }
     }
}

产生：

false
Loading TestLoaded$ClassToTest
Loaded
true

请注意，示例类不在包中。需要完整的二进制名称。

Note that the example classes are not in a package. The full binary name is required.

二进制名称的示例是java.security.KeyStore $ Builder $ FileBuilder $ 1

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