Created
April 11, 2016 18:03
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A very simple java8-style snippet to assert exception inside test method. Works without JUnit boilerplate (like rules, @expected annotations and so on) and without explicit try-catch block.
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package tech.toparvion.gist; | |
import org.junit.Test; | |
import java.io.ByteArrayInputStream; | |
import java.io.IOException; | |
import java.io.InputStream; | |
import static tech.toparvion.gist.TestHelper.assertException; | |
public class InputStreamTest { | |
@Test | |
public void testThrowing() throws Exception { | |
// SUT - system under test - a trivial input stream. | |
InputStream stream = new ByteArrayInputStream(new byte[0]); | |
// Let's make it unusable | |
stream.close(); | |
// ... and try to use: | |
final byte[] readBuf = new byte[10]; | |
// Thanks to lambdas we can either call methods as usual | |
assertException(() -> stream.read(readBuf), IOException.class); | |
// .. or through a method reference | |
assertException(stream::available, IOException.class); | |
// We'd be here just in case tested methods have really thrown excpected exceptions. | |
// Do any stuff you need further. | |
} | |
} |
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package tech.toparvion.gist; | |
import org.junit.Assert; | |
import static java.lang.String.format; | |
public class TestHelper { | |
/** | |
* Asserts that proposed <code>action</code> throws exception of <code>exceptedExceptionClass</code>. If action | |
* doesn't throw, an {@link AssertionError} is thrown. | |
* @param action tested action; can be expressed with a lambda expression | |
* @param exceptedExceptionClass a class of exception expected to be thrown | |
*/ | |
public static void assertException(TestedAction action, | |
Class<? extends Exception> exceptedExceptionClass) { | |
try { | |
action.act(); | |
Assert.fail("Tested action has not thrown any exception."); | |
} catch (Throwable e) { | |
if (!exceptedExceptionClass.isAssignableFrom(e.getClass())) { | |
Assert.fail(format("Tested action has thrown exception of class '%s' while expected class is '%s'.", | |
e.getClass().getName(), exceptedExceptionClass.getName())); | |
} | |
} | |
} | |
@FunctionalInterface | |
public interface TestedAction { | |
void act() throws Exception; | |
} | |
} |
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