(不是答案，但有一些代码注释.)

(Not an answer, but some code notes.)

1)名称函数使用小写字母，例如:

1) Name functions with a lower case letter, e.g.:

public String login() { ... }

2)使用规范Java来检查布尔值，例如:

2) Use canonical Java for checking booleans, e.g.:

if (!isValidUser) { ... }

3)使用正确命名规范代码的变量名称，例如:

3) Use variable names that make canonical code "read" properly, e.g.:

if (!validUser) { ... }

4)进行字符串比较时，无需进行空检查，例如:

4) Remove the need for null checks when doing string compares, e.g.:

if ("de".equals(getLang()) { ... }

另外，我不清楚[code> setLang()是否只是用 getLang()检索的内容的设置器；这有点令人困惑，并且在没有进一步知识的情况下使得那段代码有点奇怪.

Also, it's not clear to me if setLang() is just a setter for what's retrieved with getLang(); that's a little confusing and makes that chunk of code kind of weird without further knowledge.

5)不要将有效的用户代码放在相同的条件中；如果它是无效用户，则将其视为特殊情况并返回.然后将有效的用户代码与保护子句区分开，例如:

5) Don't put the valid user code in the same conditional; if it's an invalid user, treat it as a special case and return. The valid user code is then differentiated from the guard clause, e.g.:

if (!validUser) {
    // actionMessage stuff.
    return ERROR;
} 

session = ActionContext.getContext().getSession();
// The rest of valid user code.

6)将布尔值用于布尔值，例如:

6) Use booleans for booleans, e.g.:

session.put("loggedIn", true);

7)对范围变量使用常量，例如:

7) Use constants for scoped variables, e.g.:

session = ActionContext.getContext().getSession();
session.put(SessionKeys.LOGGED_IN, true);

8)使用 SessionAware ，它使测试更加容易；那么 session 只是一个动作属性:

8) Use SessionAware, it makes testing easier; session is then just an action property:

session.put(SessionKeys.LOGGED_IN, true);

9)当它是完全不同的东西时，不要命名 session1 ...在这种情况下，它甚至也不是什么不同，因此您要在其中输入相同的信息两次.

9) Don't name something session1 when it's a completely different thing... and in this case, it isn't even a different thing, so you're putting the same information in twice.

10)访问动作属性时要保持一致；不要使用 setXxx() 和 this.xxx .在大多数情况下，您仍然可以只使用 xxx .

10) Be consistent when accessing action properties; don't use setXxx() and this.xxx. Most of the time you can just use xxx anyway.

setCountryList(applicationManager.getCountries()); // Or, my preference in most cases:
countryList = appManager.getCountries();

11)请勿按变量类型命名，除非在需要特别区分的情况下，例如:

11) Don't name variables by their type, except in situations where you need to specifically differentiate, e.g.:

countries = appManager.getCountries();

这给我们带来了更易于管理的地方:

This leaves us with a more-manageable:

public String Login() {
    if (!appManager.isValidUser(username, password)) {
        handleInvalidUser();
        return ERROR;
    } 

    session.put(SessionKeys.LANG,          lang);
    session.put(SessionKeys.LOGGED_IN,     true);
    session.put(SessionKeys.USER_NAME,     username);
    session.put(SessionKeys.USER_PASSWORD, password);

    countries = appManager.getCountries();

    return SUCCESS;
}