无法从tomcat中自定义类加载器加载的类中获取注释

Question

给定班级org.popper.example.pages.Login

@Page(name="Login")
public interface Login {
}

导出到c:\pos\example.jar 和下面的servlet

public class PopperServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public static void main(String[] args) throws MalformedURLException, ClassNotFoundException {
        URLClassLoader ucl = new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()});
        System.out.println(Arrays.asList(ucl.loadClass("org.popper.example.pages.Login").getAnnotations()));
    }

    public PopperServlet() throws MalformedURLException, ClassNotFoundException {
        URLClassLoader ucl = new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()});
        System.out.println(Arrays.asList(ucl.loadClass("org.popper.example.pages.Login").getAnnotations()));
   }
}

以main方式运行代码显示预期结果

[@org.popper.fw.annotations.Page(name=Login)]

在tomcat中作为servlet运行代码找不到注解

[]

谁能告诉我为什么？

Answer 1

和往常一样：注意类加载器层次结构！

new URLClassLoader(new URL[] {new File("c:/pos/example.jar").toURI().toURL()}, PopperServlet.class.getClassloader());

成功了。但令人惊讶的是没有找到注释而是ClassNotFoundException 或NoClassDefError，这就是我在加载类时找不到注释时所期望的......

Answer 2

是的，你肯定找不到你的注释，你必须注释你的注释才能在运行时保持活动状态，添加：

@Retention(RetentionPolicy.RUNTIME)

Retention Java doc : 指明注解类型的注解要保留多长时间。如果注释类型声明中没有保留注释，则保留策略默认为 RetentionPolicy.CLASS。

更多详情：here

Answer 3

我也遇到了同样的问题。我通过我的自定义类加载器解决了它，也许你可以尝试一下。

类加载器代码：

import java.util.HashMap;
import java.util.Map;
/**
 * Load class from byte[] which is compiled in memory.
 *
 * @author David
 */
class CustomClassLoader extends ClassLoader {

    // class name to class bytes:
    private Map<String, byte[]> classBytes = new HashMap<String, byte[]>();

    public CustomClassLoader(Map<String, byte[]> classBytes) {
        super(CustomClassLoader.class.getClassLoader());
        Thread.currentThread().setContextClassLoader(this);
        this.classBytes.putAll(classBytes);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] buf = classBytes.get(name);
        if (buf == null) {
            return super.findClass(name);
        }
        classBytes.remove(name);
        return defineClass(name, buf, 0, buf.length);
    }

}

客户代码：

byte[] code = this.createEntity(logicalTable, keyCount, relationMap);
Map<String, byte[]> results = Maps.newHashMap();
results.put(this.entityPackage + "." + logicalTable.getTableName(), code);
CustomClassLoader classLoader = new CustomClassLoader(results);
Class<?> clazz = classLoader.findClass(this.entityPackage + "." + logicalTable.getTableName());