为什么我的单身人士 class 会抛出 StackOverflowerror？

Question

我一直在写程序。程序中的一切都由 'Engine' class 控制。因此我把它变成了单身人士。这是我当前运行良好的代码。

    package org.bautista.cybersafe.core;

import javax.swing.SwingUtilities;

import org.bautista.cybersafe.ui.MainUI;
import org.bautista.cybersafe.util.Cache;
import org.bautista.cybersafe.util.Config;
import org.bautista.cybersafe.util.account.Account;
import org.bautista.cybersafe.util.account.AccountManager;
import org.bautista.cybersafe.util.user.User;
import org.bautista.cybersafe.util.user.UserManager;

public class Engine {
    private static Engine instance;
    private AccountManager accountManager;
    private final MainUI ui;
    private final UserManager userManager;
    private final Config config;
    private User currentUser;

    private Engine() {
        instance = this; //THIS IS LINE 22
        if (!Cache.cacheExists()) {
            if (!Cache.createCache()) {
                System.out.println("Error creating cache.");
            }
        }
        config = new Config();
        userManager = new UserManager();
        ui = new MainUI();
    }

    public static Engine getInstance() {
        return instance == null ? instance = new Engine() : instance;
    }

    public void setCurrentUser(User user) {
        currentUser = user;
    }

    public User getCurrentUser() {
        return currentUser;
    }

    public AccountManager getAccountManager() {
        return accountManager;
    }

    public Config getConfig() {
        return config;
    }

    public UserManager getUserManager() {
        return userManager;
    }

    public void logOut() {
        currentUser = null;
        accountManager = null;
        ui.showLogin();
    }

    public void openAccountViewer(final Account account) {
        ui.showAccount(account);
        ui.setTitle("Cyber Safe - [" + currentUser.getUsername() + "] -"
                + account.getName());
    }

    public void openCreateAccountScreen() {
        ui.showCreateAccount();
    }

    public void openCreateUserScreen() {
        ui.showCreateUser();
    }

    public void openLoginScreen() {
        ui.showLogin();
        ui.setTitle("Cyber Safe");
    }

    public void openSafeScreen() {
        if (accountManager == null) {
            accountManager = new AccountManager(currentUser);
        }
        ui.showSafe();
        ui.setTitle("Cyber Safe - [" + currentUser.getUsername() + "]");
    }

    public void refreshUI() {
        ui.refresh();
    }

    public void updateAccountPreviews() {
        ui.updateAccountScroller();
    }

    public void run() {
        try {
            SwingUtilities.invokeAndWait(() -> ui.setVisible(true));
        } catch (final Exception e) {
            e.printStackTrace();
        }
    }

}

当我注释掉第22行时

instance = this;

我收到 Whosebugerror。当我调试程序时，我发现引擎构造函数被重复调用，就好像它在执行递归，直到我最终得到错误。为什么会这样？我的#getInstance() 方法不应该将实例作为 'Engine' class 的新实例启动吗？

这是堆栈跟踪：

Exception in thread "main" java.lang.WhosebugError
at java.io.InputStream.<init>(InputStream.java:45)
at java.io.FileInputStream.<init>(FileInputStream.java:123)
at org.bautista.cybersafe.util.Config.loadProperties(Config.java:67)
at org.bautista.cybersafe.util.Config.<init>(Config.java:29)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:28)
at org.bautista.cybersafe.core.Engine.getInstance(Engine.java:34)
at org.bautista.cybersafe.util.user.UserManager.loadUsers(UserManager.java:73)
at org.bautista.cybersafe.util.user.UserManager.<init>(UserManager.java:20)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:29)
at org.bautista.cybersafe.core.Engine.getInstance(Engine.java:34)
at org.bautista.cybersafe.util.user.UserManager.loadUsers(UserManager.java:73)
at org.bautista.cybersafe.util.user.UserManager.<init>(UserManager.java:20)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:29)
at org.bautista.cybersafe.core.Engine.getInstance(Engine.java:34)
at org.bautista.cybersafe.util.user.UserManager.loadUsers(UserManager.java:73)
at org.bautista.cybersafe.util.user.UserManager.<init>(UserManager.java:20)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:29)
at org.bautista.cybersafe.core.Engine.getInstance(Engine.java:34)
at org.bautista.cybersafe.util.user.UserManager.loadUsers(UserManager.java:73)
at org.bautista.cybersafe.util.user.UserManager.<init>(UserManager.java:20)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:29)

And here is the full project on Github

提前致谢！

Answer 1

堆栈跟踪显示以下循环：

at org.bautista.cybersafe.util.user.UserManager.loadUsers(UserManager.java:73)
at org.bautista.cybersafe.util.user.UserManager.<init>(UserManager.java:20)
at org.bautista.cybersafe.core.Engine.<init>(Engine.java:29)
at org.bautista.cybersafe.core.Engine.getInstance(Engine.java:34)

Engine.getInstance() 呼叫 new Engine().
new Engine() 呼叫 new UserManager().
new UserManager() 呼叫 UserManager.loadUsers().
UserManager.loadUsers() 调用了 Engine.getInstance()，但尚未分配 Engine.instance，因为之前的 new Engine() 调用尚未返回。

这就是为什么在调用 new UserManager() 之前在构造函数中分配 Engine.instance 可以解决问题。

您应该重新组织代码以防止初始化循环。 UserManager 和 Engine 在初始化期间不应相互依赖。

请注意，按照另一个答案中的建议执行 private static Engine instance = new Engine() 不会修复您的初始化循环。

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