替换实例化类的实现而不触及代码（java） [英] Replace implementation of instantiated class without touching the code (java)

问题描述

我有遗产代码我不想碰。

I have legacy code I don't want to touch.

public class LegacyCode{
    public LegacyCode() {
        Service s = new ClassA();
        s.getMessage();
    }
}

其中 ClassA 提供CORBA服务电话。

Where ClassA provides a CORBA service call.

public class ClassA implements Service{
    @Override
    public String getMessage() {
       // Make CORBA service call...
       return "Class A";
    }
}

接口服务看起来像;

public interface Service {
    String getMessage();
}

出于测试目的，我想替换服务（在 LegacyCode 由 ClassA 实施>），带有存根。

For test purposes, I want to replace the implementation of Service (in LegacyCode implemented by ClassA) with a stub.

public class ClassB implements Service {
    @Override
    public String getMessage() {
        return "Stub Class B";
    }
}

到目前为止一切顺利。但是，在显示的遗留代码中没有任何修改就可以在<$的实例化时加载 ClassB 而不是 ClassA C $ C> ClassA的？

So far so good. But is it possible without any modifications at the shown legacy code to load ClassB instead of ClassA at the instantiation of ClassA?

// In my test workbench
new LegacyCode(); // "Stub Class B"

我尝试编写自定义类加载器并在应用程序中加载它从java vm参数开始，但只有第一个类（这里是 LegacyCode ）由该加载器加载。

I've tried to write a custom classloader and load it at application start by java vm arguments but only the first class (here LegacyCode) was loaded by that loader.

谢谢你提前

推荐答案

使用 PowerMock 你可以创建一个模拟（或存根）对于构造函数代码。答案取自此链接。我将尝试将其转换为与您的用例完全匹配：

Using PowerMock you can create a mock (or stub) for a constructor code. The answer is taken from this link. I'll try to convert it to match exactly to your use case:

@RunWith(PowerMockRunner.class)
@PrepareForTest(ClassA.class)
public class LegacyTester {
    @Test
    public void testService() {
         // Inject your stub
         PowerMock.createMock(ClassA.class);
         Service stub = new MyServiceStub();
         PowerMock.expectNew(ClassA.class).andReturn(stub);
         PowerMock.replay(stub, ClassA.class);

         // Implement test logic here

         LegacyCode legacyCode = new LegacyCode();

         // Implement Test steps

        // Call verify if you want to make sure the ClassA constructor was called
        PowerMock.verify(stub, ClassA.class)
    }
}

这样你就可以在调用 ClassA 构造函数发生，而不更改遗留代码。希望这就是你所需要的。

This way you inject your stub when the call to ClassA constructor happens, without changing the legacy code. Hope that's what you needed.

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