System.out.println() 的 JUnit 测试 [英] JUnit test for System.out.println()

问题描述

我需要为一个旧的应用程序编写 JUnit 测试，该应用程序设计不佳并且向标准输出写入了大量错误消息.当 getResponse(String request) 方法正确运行时，它会返回一个 XML 响应:

I need to write JUnit tests for an old application that's poorly designed and is writing a lot of error messages to standard output. When the getResponse(String request) method behaves correctly it returns a XML response:

@BeforeClass
public static void setUpClass() throws Exception {
    Properties queries = loadPropertiesFile("requests.properties");
    Properties responses = loadPropertiesFile("responses.properties");
    instance = new ResponseGenerator(queries, responses);
}

@Test
public void testGetResponse() {
    String request = "<some>request</some>";
    String expResult = "<some>response</some>";
    String result = instance.getResponse(request);
    assertEquals(expResult, result);
}

但是当它出现格式错误的 XML 或无法理解请求时，它会返回 null 并将一些内容写入标准输出.

But when it gets malformed XML or does not understand the request it returns null and writes some stuff to standard output.

有没有办法在 JUnit 中断言控制台输出?捕捉以下情况:

Is there any way to assert console output in JUnit? To catch cases like:

System.out.println("match found: " + strExpr);
System.out.println("xml not well formed: " + e.getMessage());

推荐答案

using ByteArrayOutputStream 和 System.setXXX 很简单:

using ByteArrayOutputStream and System.setXXX is simple:

private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();
private final PrintStream originalOut = System.out;
private final PrintStream originalErr = System.err;

@Before
public void setUpStreams() {
    System.setOut(new PrintStream(outContent));
    System.setErr(new PrintStream(errContent));
}

@After
public void restoreStreams() {
    System.setOut(originalOut);
    System.setErr(originalErr);
}

示例测试用例:

@Test
public void out() {
    System.out.print("hello");
    assertEquals("hello", outContent.toString());
}

@Test
public void err() {
    System.err.print("hello again");
    assertEquals("hello again", errContent.toString());
}

我使用此代码来测试命令行选项(断言 -version 输出版本字符串等)

I used this code to test the command line option (asserting that -version outputs the version string, etc etc)

测试后，此答案的先前版本称为 System.setOut(null)；这是NullPointerExceptions评论者引用的原因.

Prior versions of this answer called System.setOut(null) after the tests; This is the cause of NullPointerExceptions commenters refer to.

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