java - Guice注入(inject)空指针

Question

我们尝试用 Guice 重构一个项目。这个想法是将所有语言接口(interface)绑定(bind)到一个具体的对象，比如法语或波兰语。

我们有一个绑定(bind)模块:

public class StandardModule extends AbstractModule {

    @Override
    protected void configure() {

       bind(Language.class).to(Polish.class);

    }
 }

还有一个使用这个注入(inject)对象的类(AboutDialog.java):

@Inject Language language;

public AboutDialog(JFrame parent) {
    super(parent, "", true);
    this.language=language;
    this.setTitle(language.getLanguageInUse().getString("AboutDialog.title"));
    this.parent = parent;
    try {
        jbInit();
    } catch (Exception e) {
        e.printStackTrace();
    }
    pack();
}

结果是:

java.lang.NullPointerException at net.sf.jmoney.gui.AboutDialog.<init>(AboutDialog.java:67)

第 67 行是:

this.setTitle(language.getLanguageInUse().getString("AboutDialog.title"));

我们的界面是:

public interface Language {

    public ResourceBundle getLanguageInUse();
}

波兰语课是:

public class Polish implements Language {

    private ResourceBundle languageInUse;

    public Polish() {
        languageInUse = ResourceBundle.getBundle(Constants.LANGUAGE_PL);
    }

    public ResourceBundle getLanguageInUse() {
        return languageInUse;
    }


}

我们迷路了......

Answer 1

您正在使用“字段注入(inject)”。这将使在构造函数中使用注入(inject)的值变得困难；即使 Guice 要创建对象(现在不会发生)或者您要使用 injector.injectMembers(aboutDialog)，构造函数也会在注入(inject)器有机会注入(inject)您想要的字段之前运行.

创建一个既接受可变参数又接受注入(inject)参数的类有点棘手。这给您留下了几个选择:

• 注入(inject) JFrame。如果您知道在创建构造函数时将使用什么 JFrame，则只需在您的模块中使用 bind(JFrame.class).toInstance(myJFrame);。然后 Guice 可以完全创建 AboutDialog。

• 手动创建工厂。这样你就可以注入(inject) AboutDialog.Factory 并调用 create 来获取你的 AboutDialog。它看起来像这样:

  public class AboutDialog extends JDialog {
  
    /** Injectable factory. */
    public static class Factory {
      @Inject private Language language;
  
      public AboutDialog create(JFrame parent) {
        return new AboutDialog(parent, language);
      }
    }
  
    // no @Inject parameter; you're calling "new" yourself above!
    public AboutDialog(JFrame parent, Language language) {
      super(parent, "", true);
      this.language = language;
      // ... other initialization
    }
  }
  

• 创建工厂并让 Guice 通过 assisted injection 为您连接起来.

  public class AboutDialog extends JDialog {
  
    public interface Factory {
      public AboutDialog create(JFrame parent);
    }
  
    // you need the @Inject, and also the @Assisted to tell Guice to
    // use the parameter instead of Guice bindings
    @Inject
    public AboutDialog(@Assisted JFrame parent, Language language) {
      super(parent, "", true);
      this.language = language;
      // ... other initialization
    }
  }
  
  public class StandardModule extends AbstractModule {
    @Override protected void configure() {
      bind(Language.class).to(Polish.class);
  
      // here every method in AboutDialog.Factory will be implemented
      // to create the method's return type [AboutDialog] based on
      // the parameters (like JFrame) and the bindings (like Language)
      install(new FactoryModuleBuilder().build(AboutDialog.Factory.class));
    }
  }
  

如问题评论中所述，确保您通过 @Injected 构造函数获取 AboutDialog(或 AboutDialog.Factory/field 或来自 Injector 本身，否则 Guice 将不知道注入(inject)参数。