为了让VonC在JUnit 4上运行，我修改了如下代码

protected static class ExitException extends SecurityException {
    private static final long serialVersionUID = -1982617086752946683L;
    public final int status;

    public ExitException(int status) {
        super("There is no escape!");
        this.status = status;
    }
}

private static class NoExitSecurityManager extends SecurityManager {
    @Override
    public void checkPermission(Permission perm) {
        // allow anything.
    }

    @Override
    public void checkPermission(Permission perm, Object context) {
        // allow anything.
    }

    @Override
    public void checkExit(int status) {
        super.checkExit(status);
        throw new ExitException(status);
    }
}

private SecurityManager securityManager;

@Before
public void setUp() {
    securityManager = System.getSecurityManager();
    System.setSecurityManager(new NoExitSecurityManager());
}

@After
public void tearDown() {
    System.setSecurityManager(securityManager);
}

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".

可以使用java SecurityManager防止当前线程关闭java虚拟机。下面的代码应该是你想要的:

SecurityManager securityManager = new SecurityManager() {
    public void checkPermission(Permission permission) {
        if ("exitVM".equals(permission.getName())) {
            throw new SecurityException("System.exit attempted and blocked.");
        }
    }
};
System.setSecurityManager(securityManager);

我们在代码库中使用的一个技巧是将对System.exit()的调用封装在Runnable impl中，该方法默认使用该impl。为了单元测试，我们设置了一个不同的模拟Runnable。就像这样:

private static final Runnable DEFAULT_ACTION = new Runnable(){
  public void run(){
    System.exit(0);
  }
};

public void foo(){ 
  this.foo(DEFAULT_ACTION);
}

/* package-visible only for unit testing */
void foo(Runnable action){   
  // ...some stuff...   
  action.run(); 
}

.．.以及JUnit测试方法……

public void testFoo(){   
  final AtomicBoolean actionWasCalled = new AtomicBoolean(false);   
  fooObject.foo(new Runnable(){
    public void run(){
      actionWasCalled.set(true);
    }   
  });   
  assertTrue(actionWasCalled.get()); 
}

系统存根(https://github.com/webcompere/system-stubs)也能够解决这个问题。它共享System Lambda的语法，用于包装我们知道将执行System的代码。退出，但当其他代码意外退出时，可能会导致奇怪的效果。

通过JUnit 5插件，我们可以保证任何退出都将转换为异常:

@ExtendWith(SystemStubsExtension.class)
class SystemExitUseCase {
    // the presence of this in the test means System.exit becomes an exception
    @SystemStub
    private SystemExit systemExit;

    @Test
    void doSomethingThatAccidentallyCallsSystemExit() {
        // this test would have stopped the JVM, now it ends in `AbortExecutionException`
        // System.exit(1);
    }

    @Test
    void canCatchSystemExit() {
        assertThatThrownBy(() -> System.exit(1))
            .isInstanceOf(AbortExecutionException.class);

        assertThat(systemExit.getExitCode()).isEqualTo(1);
    }
}

或者，类似断言的静态方法也可以使用:

assertThat(catchSystemExit(() -> {
   //the code under test
   System.exit(123);
})).isEqualTo(123);

实际上，您可以模拟或排除该系统。exit方法，在JUnit测试中。

例如，使用JMockit你可以这样写(也有其他方法):

@Test
public void mockSystemExit(@Mocked("exit") System mockSystem)
{
    // Called by code under test:
    System.exit(); // will not exit the program
}

编辑:替代测试(使用最新的JMockit API)，它不允许任何代码在调用System.exit(n)后运行:

@Test(expected = EOFException.class)
public void checkingForSystemExitWhileNotAllowingCodeToContinueToRun() {
    new Expectations(System.class) {{ System.exit(anyInt); result = new EOFException(); }};

    // From the code under test:
    System.exit(1);
    System.out.println("This will never run (and not exit either)");
}