从 Junit 测试调用静态单例类会引发 StackOverflow 错误

Question

当我们运行 DrawerOpenTest.java 时，它会抛出 StackOverflow 错误，而我们希望测试用例能够通过。而当 DrawerOpen 类为单例时，assertThat(actualState, is(expectedState));在测试用例中也应该是正确的。

请注意，“状态”是与 3 个基本方法的接口。

DrawerOpenTest 类

import static org.hamcrest.CoreMatchers.*;
import org.junit.*;

public class DrawerOpenTest {

@Test
public void openCloseButtonPushedPositiveTest(){
    DVDPlayer cut = DVDPlayer.getInstance(DrawerOpen.getInstance());
    State expectedState = DrawerClosedNotPlaying.getInstance();
    State actualState = cut.openCloseButtonPushed();
    assertThat(actualState, is(sameInstance(expectedState)));
}
}

DrawerOpen 类

public class DrawerOpen implements State {

private DVDPlayer player = DVDPlayer.getInstance(DrawerOpen.getInstance());

private static State state;

private DrawerOpen() {}

public static State getInstance() {
    if(state == null)
        state = new DrawerOpen();
    return state;
}


@Override
    public void openCloseButtonPushed() {
        player.close();
        player.changeState(DrawerClosedNotPlaying.getInstance());
    }

@Override
public void playButtonPushed() {
player.close();
player.play();
player.changeState(DrawerClosedPlaying.getInstance());

}

@Override
public void stopButtonPushed()
}

}

DVDPlayer 类

public class DVDPlayer {
private DVDPlayer() {}
private static DVDPlayer player = null;

private State state;

public State getState() {
    return state;
}

public static DVDPlayer getInstance(State stateParam) {
    //making it singleton
    if(player == null)
    {   
        player = new DVDPlayer();
        player.state = DrawerClosedNotPlaying.getInstance();
    }
    else 
        player.state = stateParam;
    return player;      
}

public void changeState(State newState) {
    this.state=newState;        
}
public State openCloseButtonPushed(){
    state.openCloseButtonPushed();
    return state;
}

public State playButtonPushed() {
    state.playButtonPushed();
    return state;
}

public State stopButtonPushed() {
    state.stopButtonPushed();
    return player.state;
}
public void open() {
    System.out.println("DVDPlayer is opening.....");
}
public void close() {
    System.out.println("DVDPlayer is closing.....");

}
public void play() {
    System.out.println("DVDPlayer is playing.....");
}   
public void stop() {
    System.out.println("DVDPlayer is stopping.....");
}
}

结果：它会导致 StackOverflowError，因为它会不断初始化自身。请帮助如何使该测试成功通过。

java.lang.StackOverflowError
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)
    at DrawerOpen.getInstance(DrawerOpen.java:13)
    at DrawerOpen.<init>(DrawerOpen.java:5)

Answer 1

这个：

public class DrawerOpen implements State {

  private DVDPlayer player = DVDPlayer.getInstance(DrawerOpen.getInstance());

  // ...
}

表示每次尝试创建DrawerOpen时都会调用DrawerOpen.getInstance()，因为player是成员变量。

为了在DrawerOpen.getInstance() 中创建实例，您必须创建DrawerOpen 的实例；但是再次调用DrawerOpen.getInstance()，需要你创建一个DrawerOpen，它调用DrawerOpen.getInstance()...

您可以通过将DrawerOpen.getInstance() 分配给静态字段来避免这种递归调用：

public class DrawerOpen implements State {
  private static DrawerOpen INSTANCE = DrawerOpen.getInstance();

  private DVDPlayer player = DVDPlayer.getInstance(INSTANCE);

  // ...
}

但是，还有一个问题，就是你在调用DVDPlayer.getInstance(INSTANCE)的时候并没有分配INSTANCE，所以你最终会调用DVDPlayer.getInstance(null)。

解决这个问题的一种方法是调用：

  private DVDPlayer player = DVDPlayer.getInstance(this);

但是，这是所谓的不安全发布的一个示例，其中您在DrawerOpen 类完全初始化之前泄漏了this 引用。这可能会导致更多意想不到的问题。

---

您的代码有点混乱。您应该仔细考虑一下为什么要使用单例，因为在我看来这些不像是单例属性。

例如，你有一个依赖于参数的“单例”有点狡猾：如果你用不同的State 实例调用DVDPlayer.getInstance，你当前忽略新的状态实例，并使用前一个。这很可能会导致令人困惑或令人惊讶的行为：我给了你 this 状态实例，但 DVDPlayer 正在使用 那个 状态实例。

Answer 2

改变这两行：

private DVDPlayer player = DVDPlayer.getInstance(DrawerOpen.getInstance());

private DrawerOpen() {}

到这里：

private DVDPlayer player;

private DrawerOpen() {
    player = DVDPlayer.getInstance(this);
}

这将摆脱递归初始化循环。

解释：构造函数中的this 指的是正在构造的对象。这是DrawerOpen.getInstance() 返回的单例 ...如果对象是在代码中的那个点构建的。