通过反射或其他方式覆盖java final 方法？

Question

在尝试编写测试用例时会出现这个问题。 Foo 是框架库中的一个类，我没有源代码访问权限。

public class Foo{
  public final Object getX(){
  ...
  }
}

我的申请将

public class Bar extends Foo{
  public int process(){
    Object value = getX();
    ...
  }
}

单元测试用例无法初始化，因为由于其他依赖关系，我无法创建 Foo 对象。 BarTest 抛出一个空指针，因为值为空。

public class BarTest extends TestCase{
  public testProcess(){
    Bar bar = new Bar();        
    int result = bar.process();
    ...
  }
}

有没有办法我可以使用反射 api 将 getX() 设置为非最终的？或者我应该如何进行测试？

Answer 1

因为这是 google 中“覆盖最终方法 java”的最佳结果之一。我想我会留下我的解决方案。这个类展示了一个使用示例“Bagel”类和一个免费使用的 javassist 库的简单解决方案：

/**
 * This class shows how you can override a final method of a super class using the Javassist's bytecode toolkit
 * The library can be found here: http://jboss-javassist.github.io/javassist/
 * 
 * The basic idea is that you get the super class and reset the modifiers so the modifiers of the method don't include final.
 * Then you add in a new method to the sub class which overrides the now non final method of the super class.
 * 
 * The only "catch" is you have to do the class manipulation before any calls to the class happen in your code. So put the
 * manipulation as early in your code as you can otherwise you will get exceptions.
 */

package packagename;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;
import javassist.Modifier;

/** 
 * A simple class to show how to use the library
 */
public class TestCt {

    /** 
     * The starting point for the application
     */
    public static void main(String[] args) {

        // in order for us to override the final method we must manipulate the class using the Javassist library.
        // we need to do this FIRST because once we initialize the class it will no longer be editable.
        try
        {
            // get the super class
            CtClass bagel = ClassPool.getDefault().get("packagename.TestCt$Bagel");

            // get the method you want to override
            CtMethod originalMethod = bagel.getDeclaredMethod("getDescription");

            // set the modifier. This will remove the 'final' modifier from the method.
            // If for whatever reason you needed more than one modifier just add them together
            originalMethod.setModifiers(Modifier.PUBLIC);

            // save the changes to the super class
            bagel.toClass();

            // get the subclass
            CtClass bagelsolver = ClassPool.getDefault().get("packagename.TestCt$BagelWithOptions");

            // create the method that will override the super class's method and include the options in the output
            CtMethod overrideMethod = CtNewMethod.make("public String getDescription() { return super.getDescription() + \" with \" + getOptions(); }", bagelsolver);

            // add the new method to the sub class
            bagelsolver.addMethod(overrideMethod);

            // save the changes to the sub class
            bagelsolver.toClass();
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

        // now that we have edited the classes with the new methods, we can create an instance and see if it worked

        // create a new instance of BagelWithOptions
        BagelWithOptions myBagel = new BagelWithOptions();

        // give it some options
        myBagel.setOptions("cheese, bacon and eggs");

        // print the description of the bagel to the console.
        // This should now use our new code when calling getDescription() which will include the options in the output.
        System.out.println("My bagel is: " + myBagel.getDescription());

        // The output should be:
        // **My bagel is: a plain bagel with cheese, bacon and eggs**
    }

    /**
     * A plain bagel class which has a final method which we want to override
     */
    public static class Bagel {

        /**
         * return a description for this bagel
         */
        public final String getDescription() {
            return "a plain bagel";
        }
    }

    /**
     * A sub class of bagel which adds some extra options for the bagel.
     */
    public static class BagelWithOptions extends Bagel {

        /**
         * A string that will contain any extra options for the bagel
         */
        String  options;

        /**
         * Initiate the bagel with no extra options
         */
        public BagelWithOptions() {
            options = "nothing else";
        }

        /**
         * Set the options for the bagel
         * @param options - a string with the new options for this bagel
         */
        public void setOptions(String options) {
            this.options = options;
        }

        /**
         * return the current options for this bagel
         */
        public String getOptions() {
            return options;
        }
    }
}

Answer 2

public class Bar extends Foo{
  public int process(){
    Object value = getX();
    return process2(value);
  }
  public int process2(Object value){
  ...
  }
}

public class BarTest extends TestCase{
  public testProcess(){
    Bar bar = new Bar();   
    Mockobj mo = new Mockobj();     
    int result = bar.process2(mo);
    ...
  }
}

我最终所做的就是上述内容。有点难看……詹姆斯的解决方案肯定比这个好很多……

Answer 3

如果getX() 返回的变量不是final，您可以使用What’s the best way of unit testing private methods? 中介绍的技术通过Reflection 更改private 变量的值。

Answer 4

您可以创建另一个可以在测试中覆盖的方法：

public class Bar extends Foo {
  protected Object doGetX() {
    return getX();
  }
  public int process(){
    Object value = doGetX();
    ...
  }
}

那么，您可以在 BarTest 中覆盖 doGetX。

Answer 5

Seb 是正确的，只是为了确保您得到问题的答案，没有在本机代码中做某事（我很确定这不会起作用）或在运行时修改类的字节码，并创建在运行时覆盖该方法的类，我看不到改变方法“最终性”的方法。反射在这里对你没有帮助。

Answer 6

如果您的单元测试用例由于其他依赖关系而无法创建 Foo，这可能表明您一开始就没有正确地进行单元测试。

单元测试旨在在生产代码运行的相同环境下进行测试，因此我建议在您的测试中重新创建相同的生产环境。否则，您的测试将无法完成。