Mocking JRE classes problem - mocking java.lang.Math

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Why mocking of jre (system) classes works different than mocking of other (non 
jre) ?

1.I am testing MathUserr class. MathUserr calls static method 
OtherClass.myRandom().
2.To mock OtherClass.myRandom() i need to:
2.1. add @PrepareForTest({ OtherClass.class}) annotation 
2.2. call       PowerMockito.mockStatic(OtherClass.class);
2.3. declare stubbing: when(OtherClass.myRandom()).thenReturn(2.0);
3.Now if class under test (MathUserr) calls OtherClass.myRandom(), it gets 2.0d 
value.

OK... it is clear. but now I describe the way, we mock JRE's java.lang.Math 
class.

1.I am testing MathUserr class. MathUserr calls static method Math.random();
2.To mock Math.random() i need to:
2.1. add @PrepareForTest({ MathUserr.class}) annotation  (WOOW, why MathUserr 
instead of Math???)
2.2. call       PowerMockito.mockStatic(Math.class);
2.3. declare stubbing: when(Math.random()).thenReturn(2.0);
3.Now if class under test (MathUserr) calls OtherClass.myRandom(), it gets 2.0d 
value.

Everything in the same way, except of point 2.1.

Why it works in this way ? Is it documented ? 
Thanks in advance for any information :)

Source code that I am desribing:
#############################################SOURCE CODE START####
package tp.check;

import static org.junit.Assert.*;
import static org.mockito.BDDMockito.*;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ OtherClass.class, MathUserr.class })
public class StaticTests {
    @Test
    public void mockMyStaticMethod() throws Exception {
        // given 
        MathUserr u = new MathUserr();
        PowerMockito.mockStatic(OtherClass.class);
        when(OtherClass.myRandom()).thenReturn(2.0);

        // when
        double fromOther = u.myClassRandom();

        // then
        assertEquals(2.0, fromOther, 0.0001);
    }

    @Test
    public void mockJreLibStaticMethod() throws Exception {
        // given 
        MathUserr u = new MathUserr();
        PowerMockito.mockStatic(Math.class);
        when(Math.random()).thenReturn(2.0);

        // when
        double fromOther = u.jreRandom();

        // then
        assertEquals(2.0, fromOther, 0.0001);
    }
}

class MathUserr{
    public double jreRandom(){
        return Math.random();
    }

    public double myClassRandom(){
        return OtherClass.myRandom();
    }
}

class OtherClass{
    public static double myRandom(){
        return 22.0; // ;)
    }
}

#############################################SOURCE CODE END####

        I attach my env (from maven pom) below:

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-all</artifactId>
            <version>1.8.5</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-module-junit4</artifactId>
            <version>1.4.6</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.powermock</groupId>
            <artifactId>powermock-api-mockito</artifactId>
            <version>1.4.6</version>
            <scope>test</scope>
        </dependency>

Original issue reported on code.google.com by michal.r...@gmail.com on 3 Feb 2011 at 3:39

[GoogleCodeExporter]

This is because it's impossible to byte-code manipulate Java System classes 
such as Math. It's documented here:

http://code.google.com/p/powermock/wiki/MockSystem
http://blog.jayway.com/2009/05/17/mocking-static-methods-in-java-system-classes/

Original comment by johan.ha...@gmail.com on 4 Feb 2011 at 9:32

[GoogleCodeExporter]

Original comment by johan.ha...@gmail.com on 4 Feb 2011 at 9:33

• Changed state: WontFix

[GoogleCodeExporter]

thanks a lot for your reply.

Original comment by michal.r...@gmail.com on 4 Feb 2011 at 9:38