动态地将数组项映射到 java.reflect.Field 对象

Question

我需要编写代码来模拟在运行时类中具有用户定义数量的可用字段。这个想法是能够将指向这些“动态”字段的 java.reflect.Field 对象返回给客户端代码。

class DynamicFieldClass {
 /**
  * fieldNames is the list of names of the fields we want to "exist" in the class
  * they will all be of the same type (say String)
  */
 public DynamicFieldClass(List<String> fieldNames) {
 // ... what do we do here
 }
 public Field getFieldObjectFor(String desiredFieldName) {
 // ... what do we do here
 }
}

有没有类似于 DynamicProxy 的东西（但是对于字段）？ 谢谢

Answer 1

我最终使用 Javassist 来：
- 在运行时创建一个继承自原始类的新类定义
- 将我需要的字段注入新的类定义中

我还用静态工厂方法替换了公共构造函数，该方法生成并返回新类定义的实例。总而言之，代码如下所示：

class DynamicFieldClass {
protected DynamicFieldClass()   {
}    
public Field getFieldObjectFor(String desiredFieldName) {
    return null;
}

/**
* fieldNames is the list of names of the fields we want to "exist" in the class
* they will all be of the same type (say String)
*/
public static createInstance (List<String> fieldNames)  {
    ClassPool defaultClassPool = ClassPool.getDefault(); 
    CtClass originalClass = defaultClassPool.get("DynamicFieldClass");
    CtClass newClass = defaultClassPool.makeClass("modified_DynamicFieldClass", originalClass);

    StringBuilder getterCore = new StringBuilder();        
    for (String item : fieldNames)  {
        CtField addedField = CtField.make(String.format("private String %s;", item), newClass);
        newClass.addField(addedField);
        getterCore.append(String.format("if \"%s\".equals(%1) { return this.class.getDeclaredField(%s);}", item, item));
    }            
    getterCore.append("throw new IllegalArgumentException(\"Unknown field name: \" + $1);");
    final String sourceGeneralGetter = String.format("{%s}", getterCore.toString());
    CtMethod mold = originalClass.getDeclaredMethod("getFieldObjectFor");
    CtMethod copiedMeth = CtNewMethod.copy(mold, newClass, null);
    newClass.addMethod(copiedMeth);
    CtMethod getMeth = newClass.getDeclaredMethod("getFieldObjectFor");
    getMeth.setBody(sourceGeneralGetter);
    CtConstructor defaultCtor = new CtConstructor(new CtClass[0], newClass);
    defaultCtor.setBody("{}");
    newClass.addConstructor(defaultCtor);
    Class modifiedClass = newClass.toClass();
    return modifiedClass.newInstance();
}

}