概览

为了确保我们的错误处理逻辑正常工作，验证一段代码在某些条件下是否出发特定异常是有必要的。

断言抛出异常

断言一段代码抛出特定的异常可通过使用JUnit 5中的assertThrows()方法来完成：

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@Test
void notEnoughFunds() {
    BankAccount account = new BankAccount(9);
    assertThrows(NotEnoughFundsException.class, () -> account.withdraw(10),
            "Balance must be greater than amount of withdrawal");
}

在此示例中，如果我们尝试从帐户余额允许的银行帐户中提取更多资金，则相关代码实现会抛出NotEnoughFundsException异常。

失败的断言

现在，假设在我们的示例中忘记在提款前检查余额，如果不抛出异常，测试将失败并显示错误消息：

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Balance must be greater than amount of withdrawal ==> Expected com.arhohuttunen.junit5.exception.NotEnoughFundsException to be thrown, but nothing was thrown.
org.opentest4j.AssertionFailedError: Balance must be greater than amount of withdrawal ==> Expected com.arhohuttunen.junit5.exception.NotEnoughFundsException to be thrown, but nothing was thrown.

此外，假设在我们的示例中我们忘记初始化余额，代码将抛出NullPointerException异常，如果抛出非预期异常，测试将失败并显示不同的错误消息：

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Balance must be greater than amount of withdrawal ==> Unexpected exception type thrown ==> expected: <com.arhohuttunen.junit5.exception.NotEnoughFundsException> but was: <java.lang.NullPointerException>
org.opentest4j.AssertionFailedError: Balance must be greater than amount of withdrawal ==> Unexpected exception type thrown ==> expected: <com.arhohuttunen.junit5.exception.NotEnoughFundsException> but was: <java.lang.NullPointerException>

断言异常消息

此外，有时我们想要验证异常的一些信息，例如错误消息或原因，在这种情况下可以捕获抛出的异常：

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@Test
void notEnoughFundsWithMessage() {
    BankAccount account = new BankAccount(0);
    Throwable thrown = assertThrows(NotEnoughFundsException.class, () -> account.withdraw(100));
    assertEquals("Attempted to withdraw 100 with a balance of 0", thrown.getMessage());
}

总结

JUnit 5可以轻松断言使用assertThrows()方法引出发预期的异常，此外也可以捕获抛出的异常以检查错误消息等更多信息。

本文的示例代码能在GitHub中找到。