I am running into a problem mocking out java.lang.Math
using JMockit (1.21). See below for a simplified use of my actual class. Basically somewhere in my code I use Math.pow(...)
and I whish to mock it.
public final class Calculator {
public final double power(double base, double exponent) {
return Math.pow(base, exponent);
}
}
As for my test code, this test works.
@Test
public void simpleTestWithoutMock() {
Calculator calculator = new Calculator();
double result = calculator.power(2, 3);
assertThat(result).isEqualTo(8);
}
This test fails.
@Test
public void simpleTestWithMock(@Mocked Math math) {
new Expectations() {{ // This would be line 67 in below stracktrace
Math.pow(2,3); result = 1;
}};
Calculator calculator = new Calculator();
double result = calculator.power(2, 3);
assertThat(result).isEqualTo(1);
}
The error message I get:
java.lang.NoClassDefFoundError: nl/endran/sandbox/CalculatorTest$1
at nl.endran.sandbox.CalculatorTest.simpleTestWithMock(CalculatorTest.java:67)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: java.lang.ClassNotFoundException: nl.endran.sandbox.CalculatorTest$1
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 9 more
I am able to mock other system classes like System
and RunTime
like this, but Math
just doesn't seem to work (nor does String
for that matter). I know how to circumvent this is my tests, so I am not blocked or anything, but I cannot understand why Math
cannot be mocked. Removing the Expectations block will not cause the NoClassDefFoundError
error anymore. I suspect methods that have a signature like public static native
to play a role in this, but I cannot find anything.
Because 1) methods from java.lang.Math
(in particular, Math.min(a, b)
) get called all the time by other classes from the JRE (for example, look inside java.lang.String
), and 2) when JMockit mocks a class (with @Mocked
, anyway), it gets mocked for the duration of the test, regardless of where the calls are coming from.
Bottom line, don't mock low-level JRE classes unless you really know what you are doing. In this particular case, JMockit should probably deny the full mocking of java.lang.Math
, as really there is no good reason (that I can see) for doing it.