如何使用JUnit来测试某些代码是否抛出异常?

我当然可以这样做:

@Test
public void testFooThrowsIndexOutOfBoundsException() {
  boolean thrown = false;

  try {
    foo.doStuff();
  } catch (IndexOutOfBoundsException e) {
    thrown = true;
  }

  assertTrue(thrown);
}

我记得,对于这类情况,有一个注释或Assert.xyz之类的东西远没有JUnit那么笨拙,更符合JUnit的精神。


当前回答

try {
    my method();
    fail( "This method must thrwo" );
} catch (Exception ex) {
    assertThat(ex.getMessage()).isEqual(myErrormsg);
}

其他回答

JUnit框架具有assertThrows()方法:

ArithmeticException exception = assertThrows(ArithmeticException.class, () ->
    calculator.divide(1, 0));
assertEquals("/ by zero", exception.getMessage());

对于JUnit 5,它位于org.unit.jupiter.api.Assertions类中;对于JUnit 4.13,它位于org.JUnit.Assert类中;对于JUnit4的早期版本:只需将org.JUnit.jupiter:junitjupiterapi上的引用添加到项目中,就可以从JUnit5获得非常好的工作版本。

BDD风格解决方案:JUnit 4+Catch异常+AssertJ

import static com.googlecode.catchexception.apis.BDDCatchException.*;

@Test
public void testFooThrowsIndexOutOfBoundsException() {

    when(() -> foo.doStuff());

    then(caughtException()).isInstanceOf(IndexOutOfBoundsException.class);

}

依赖关系

eu.codearte.catch-exception:catch-exception:2.0
try {
    my method();
    fail( "This method must thrwo" );
} catch (Exception ex) {
    assertThat(ex.getMessage()).isEqual(myErrormsg);
}

如前所述,JUnit中有许多处理异常的方法。但在Java8中还有另一个:使用Lambda表达式。使用Lambda表达式,我们可以实现如下语法:

@Test
public void verifiesTypeAndMessage() {
    assertThrown(new DummyService()::someMethod)
            .isInstanceOf(RuntimeException.class)
            .hasMessage("Runtime exception occurred")
            .hasMessageStartingWith("Runtime")
            .hasMessageEndingWith("occurred")
            .hasMessageContaining("exception")
            .hasNoCause();
}

assertThrown接受一个函数接口,它的实例可以用lambda表达式、方法引用或构造函数引用创建。assertThrown接受该接口将期望并准备好处理异常。

这是一种相对简单但功能强大的技术。

看看这篇描述这一技巧的博文:http://blog.codeleak.pl/2014/07/junit-testing-exception-with-java-8-and-lambda-expressions.html

源代码可在此处找到:https://github.com/kolorobot/unit-testing-demo/tree/master/src/test/java/com/github/kolorobot/exceptions/java8

披露:我是博客和项目的作者。

Java 8解决方案

如果您想要一个解决方案:

利用Java 8 lambda不依赖任何JUnit魔法允许您检查单个测试方法中的多个异常检查测试方法中特定的一组行(而不是整个测试方法中的任何未知行)是否引发异常生成引发的实际异常对象,以便您可以进一步检查它

下面是我编写的实用函数:

public final <T extends Throwable> T expectException( Class<T> exceptionClass, Runnable runnable )
{
    try
    {
        runnable.run();
    }
    catch( Throwable throwable )
    {
        if( throwable instanceof AssertionError && throwable.getCause() != null )
            throwable = throwable.getCause(); //allows testing for "assert x != null : new IllegalArgumentException();"
        assert exceptionClass.isInstance( throwable ) : throwable; //exception of the wrong kind was thrown.
        assert throwable.getClass() == exceptionClass : throwable; //exception thrown was a subclass, but not the exact class, expected.
        @SuppressWarnings( "unchecked" )
        T result = (T)throwable;
        return result;
    }
    assert false; //expected exception was not thrown.
    return null; //to keep the compiler happy.
}

(摘自我的博客)

使用方法如下:

@Test
public void testMyFunction()
{
    RuntimeException e = expectException( RuntimeException.class, () -> 
        {
            myFunction();
        } );
    assert e.getMessage().equals( "I haz fail!" );
}

public void myFunction()
{
    throw new RuntimeException( "I haz fail!" );
}