背景

程式碼設計並不總是盡善盡美，有些單元測試是Mockito力不從心的。
這時候就需要PowerMock ，這個框架是對Mockito增強。
下面展示幾個常用場景
- mock靜態方法
- 跳過私有方法
- 更改子類無法訪問的父類私有field
- 更改類的私有static常量
- 模擬New建構函式

示例

首先引入Maven依賴

        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>
 
powermock-api-mockito</artifactId>
            <version>1.7.0</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-module-junit4</artifactId
 
>
            <version>1.7.0</version>
            <scope>test</scope>
        </dependency>

要測試的業務類包含了幾個特殊用例

abstract class MyParent {

    private String parentValue = "parent";

    private String getMessageContent() {
        return parentValue;
    }

    protected
 
 String getParentMessage() {
        return getMessageContent();
    }

}

public class Mytest extends MyParent {

    private static String BID = "999";

    public Mytest() {

    }

    public static String getBid() {
        return BID;
    }

    public static String getMessage() {
        return "hello";
    }

    public String sendEmail() {
        String response = "send email";
        System.out.println(response);
        response = sendMessage("success");
        return response;
    }

    private String sendMessage(String status) {
        System.out.println("send message " + status);
        return "send message " + status;
    }
}

編寫單元測試

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.api.support.membermodification.MemberModifier;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import org.powermock.reflect.Whitebox;


@RunWith(PowerMockRunner.class) //以PowerMock啟動Junit test
@PrepareForTest({Mytest.class}) //預處理要mock的類，在位元組碼的級別更改類的行為
public class Testmethod {
    @Test
    public void testStaticMethod() {//mock靜態方法
        PowerMockito.mockStatic(Mytest.class);
        Mockito.when(Mytest.getMessage()).thenReturn("world");
        Assert.assertEquals("world", Mytest.getMessage());
    }

    @Test
    public void skipPrivateMethod() {//跳過私有方法
        Mytest t = new Mytest();
        PowerMockito.suppress(PowerMockito.method(Mytest.class, "sendMessage", String.class));  //類，方法名，引數類
        String response = t.sendEmail();
        Assert.assertTrue(response == null);//跳過了sendMessage方法，返回為null
    }

    @Test
    public void changeParentPrivate() {//更改子類無法訪問的父類私有field
        Mytest t = new Mytest();
        try {
            MemberModifier.field(Mytest.class, "parentValue").set(
                    t, "tomcat");
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        Assert.assertEquals("tomcat", t.getParentMessage());//父類私有field已經被更改
    }

    @Test
    public void changeBid() {//更改類的私有static常量
        Whitebox.setInternalState(Mytest.class, "BID", "888");
        Assert.assertEquals("888", Mytest.getBid());
    }

    @Test
    public void mockNew() throws Exception {//模擬New構造方法
        Mytest test = Mockito.mock(Mytest.class);
        Mockito.when(test.getParentMessage()).thenReturn("hello");

        PowerMockito.whenNew(Mytest.class).withNoArguments().thenReturn(test);

        Mytest testNew = new Mytest();

        Assert.assertEquals("hello", testNew.getParentMessage());
    }
}

已知問題

PowerMock實現了自己的類載入器，佔用記憶體大，容易造成記憶體不足:OutOfMemoryError