我喜欢已经给出的一些答案，但我想演示一种不同的技术，这种技术在测试遗留代码时通常很有用。给出如下代码:

public class Foo {
  public void bar(int i) {
    if (i < 0) {
      System.exit(i);
    }
  }
}

您可以进行安全重构来创建包装系统的方法。退出电话:

public class Foo {
  public void bar(int i) {
    if (i < 0) {
      exit(i);
    }
  }

  void exit(int i) {
    System.exit(i);
  }
}

然后你可以为你的测试创建一个覆盖exit的假的:

public class TestFoo extends TestCase {

  public void testShouldExitWithNegativeNumbers() {
    TestFoo foo = new TestFoo();
    foo.bar(-1);
    assertTrue(foo.exitCalled);
    assertEquals(-1, foo.exitValue);
  }

  private class TestFoo extends Foo {
    boolean exitCalled;
    int exitValue;
    void exit(int i) {
      exitCalled = true;
      exitValue = i;
    }
}

这是一种用行为代替测试用例的通用技术，我在重构遗留代码时一直使用它。这通常不是我要留下东西的地方，而是测试现有代码的中间步骤。

Another technique here is to introduce additional code into the (hopefully small number of) places where the logic does the System.exit(). This additional code then avoids doing the System.exit() when the logic is being called as part of unit test. For example, define a package private constant like TEST_MODE which is normally false. Then have the unit test code set this true and add logic to the code under test to check for that case and bypass the System.exit call and instead throw an exception that the unit test logic can catch. By the way, in 2021 and using something like spotbugs, this problem can manifest itself in the obscure error that "java.io.IOException: An existing connection was forcibly closed by the remote host".

你可以用替换Runtime实例来测试System.exit(..)。 例如，使用TestNG + Mockito:

public class ConsoleTest {
    /** Original runtime. */
    private Runtime originalRuntime;

    /** Mocked runtime. */
    private Runtime spyRuntime;

    @BeforeMethod
    public void setUp() {
        originalRuntime = Runtime.getRuntime();
        spyRuntime = spy(originalRuntime);

        // Replace original runtime with a spy (via reflection).
        Utils.setField(Runtime.class, "currentRuntime", spyRuntime);
    }

    @AfterMethod
    public void tearDown() {
        // Recover original runtime.
        Utils.setField(Runtime.class, "currentRuntime", originalRuntime);
    }

    @Test
    public void testSystemExit() {
        // Or anything you want as an answer.
        doNothing().when(spyRuntime).exit(anyInt());

        System.exit(1);

        verify(spyRuntime).exit(1);
    }
}

快速查看api，可以看到System。Exit可以抛出异常，特别是当securitymanager禁止关闭vm时。也许一个解决方案是安装这样一个管理器。

如何注入一个“ExitManager”到这个方法:

public interface ExitManager {
    void exit(int exitCode);
}

public class ExitManagerImpl implements ExitManager {
    public void exit(int exitCode) {
        System.exit(exitCode);
    }
}

public class ExitManagerMock implements ExitManager {
    public bool exitWasCalled;
    public int exitCode;
    public void exit(int exitCode) {
        exitWasCalled = true;
        this.exitCode = exitCode;
    }
}

public class MethodsCallExit {
    public void CallsExit(ExitManager exitManager) {
        // whatever
        if (foo) {
            exitManager.exit(42);
        }
        // whatever
    }
}

生产代码使用ExitManagerImpl，测试代码使用ExitManagerMock，可以检查是否调用了exit()以及使用哪个退出代码。

使用运行时。exec(字符串命令)在单独的进程中启动JVM。