Testing Mastery: JUnit, Mockito & Integration Tests · 课时

异常测试与超时

验证预期异常，并实现超时机制，防止无限循环或运行时间过长的测试用例。

异常测试与超时 是 CoddyKit 上的免费 Testing Mastery: JUnit, Mockito & Integration Tests 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Testing Mastery: JUnit, Mockito & Integration Tests 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Testing Mastery: JUnit, Mockito & Integration Tests 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Test for Expected Errors

Sometimes, your code is designed to throw an error under specific, invalid conditions. For example, dividing by zero. Exception testing ensures that your code handles these situations correctly.

It validates both the success and failure paths.
It confirms your error messages are accurate.

Catching Exceptions with JUnit

JUnit 5 provides the assertThrows method to verify that a specific exception is thrown. This is the recommended and cleanest way to test for expected exceptions.

It takes the expected exception type (e.g., IllegalArgumentException.class).
It takes a lambda expression (a small piece of code) that should trigger the exception.

`assertThrows` in Action

Let's see assertThrows in a simple test. Our Calculator method divide should throw an IllegalArgumentException if the divisor is zero.

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class Calculator {
    public int divide(int a, int b) {
        if (b == 0) {
            throw new IllegalArgumentException("Divisor cannot be zero");
        }
        return a / b;
    }
}

public class ExceptionTest {
    @Test
    void testDivideByZeroThrowsException() {
        Calculator calc = new Calculator();
        assertThrows(IllegalArgumentException.class, () -> {
            calc.divide(10, 0);
        });
    }
}

Checking Exception Details

It's often not enough to just know *that* an exception was thrown. You might also want to verify its details, like the exception message or other properties.

The assertThrows method returns the actual exception object, allowing you to perform further assertions on it.

Asserting Exception Message

Here, we capture the exception object returned by assertThrows and then use assertEquals to verify its message. This makes your exception tests very precise.

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class Calculator {
    public int divide(int a, int b) {
        if (b == 0) {
            throw new IllegalArgumentException("Divisor cannot be zero");
        }
        return a / b;
    }
}

public class ExceptionMessageTest {
    @Test
    void testDivideByZeroMessage() {
        Calculator calc = new Calculator();
        IllegalArgumentException thrown = assertThrows(
            IllegalArgumentException.class,
            () -> calc.divide(10, 0),
            "Expected divide() to throw, but it didn't"
        );
        assertEquals("Divisor cannot be zero", thrown.getMessage());
    }
}

Preventing Endless Tests with Timeouts

Sometimes, a piece of code can get stuck in an infinite loop or take an unexpectedly long time to complete. This can cause your test suite to hang indefinitely.

Timeouts are a crucial feature that automatically fail a test if it exceeds a specified duration, ensuring your test suite runs efficiently.

Setting Timeouts with `@Timeout`

JUnit 5 uses the @Timeout annotation to set a maximum duration for test methods or entire test classes. You specify the maximum value and a unit of time.

Common units include TimeUnit.SECONDS, TimeUnit.MILLISECONDS, TimeUnit.MINUTES.
If the test runs longer than the specified time, it will fail.

Method-Level Timeout Example

Here's how to apply a timeout to a single test method. If the longRunningTask() takes more than 1 second, this test will automatically fail.

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.Timeout;
import java.util.concurrent.TimeUnit;
import static org.junit.jupiter.api.Assertions.*;

public class TimeoutMethodTest {

    // A method that might take too long
    private void longRunningTask() throws InterruptedException {
        Thread.sleep(1500); // Simulates a task taking 1.5 seconds
    }

    @Test
    @Timeout(value = 1, unit = TimeUnit.SECONDS)
    void testTaskShouldCompleteWithinOneSecond() throws InterruptedException {
        longRunningTask(); // This will cause the test to fail due to timeout
        assertTrue(true, "Task completed (but it will timeout)");
    }
}

Class-Level Timeouts

You can also apply the @Timeout annotation at the class level. When placed on a test class, all test methods within that class will inherit the specified timeout.

If a test method also has its own @Timeout annotation, the method-level timeout will override the class-level one for that specific method.

Test Your Knowledge

Which JUnit 5 feature is best suited to ensure a test method completes execution within a predefined time limit, failing if it exceeds that limit?

Recap: Exceptions & Timeouts

In this lesson, you learned to write robust tests for expected errors and to prevent runaway tests:

You used assertThrows to verify that specific exceptions are thrown by your code.
You learned how to verify specific details of an exception, such as its message.
You applied the @Timeout annotation to set time limits for your test methods and classes, ensuring efficient test execution.

常见问题解答

「异常测试与超时」课时是免费的吗？

是的 — 「异常测试与超时」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Testing Mastery: JUnit, Mockito & Integration Tests 课程的其余内容，请升级到 CoddyKit PRO。 Testing Mastery: JUnit, Mockito & Integration Tests 课程共包含 4 节课。

「异常测试与超时」这节课中我会学到什么？

验证预期异常，并实现超时机制，防止无限循环或运行时间过长的测试用例。你通过在浏览器中直接运行的动手代码来练习 Testing Mastery: JUnit, Mockito & Integration Tests，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Testing Mastery: JUnit, Mockito & Integration Tests 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Testing Mastery: JUnit, Mockito & Integration Tests 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 3 节课，共 4 节。

「异常测试与超时」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

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