java - 无法使用我的列表初始化类

Question

我无法在下面的类中摆脱这个 NullPointerException 。我的 Controller 正在扩展此类，当我注释掉下面的构造函数时，问题就消失了......我做错了什么？

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'contentController' defined in file [C:\Users\bkuhl\JavaProjects\cmt\cmt\target\cmt\WEB-INF\classes\com\site\cmt\web\ContentController.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.fettergroup.cmt.web.ContentController]: Constructor threw exception; nested exception is java.lang.NullPointerException

<小时/>

public class Controller {
    private List<String> cssFiles;
    private List<String> jsFiles;

    public Controller () {
        this.addCss("global.css");
        this.addJs("global.js");
    }

    public ModelAndView render (ModelAndView model) {

        //add Js & Css files
        model.addObject("cssFiles", this.cssFiles);
        model.addObject("jsFiles", this.jsFiles);

        return model;
    }

    /*
     * Add a css file to the page
     */
    public final void addCss (String cssPath) {
        cssFiles.add(this.cssFiles.size(), cssPath);
    }

    /*
     * Add a javascript file to the page
     */
    public final void addJs (String jsPath) {
        jsFiles.add(this.jsFiles.size(), jsPath);
    }
}

Answer 1

cssFiles 和 jsFiles 从未显式初始化，因此为 null。您应该使用 List 实现(例如 ArrayList)来初始化它们。

private List<String> cssFiles = new ArrayList<String>();
private List<String> jsFiles = new ArrayList<String>();

当您声明字段(类变量)但未对其进行赋值时，该值默认为 null(对于引用)、0(对于数字)或 false( boolean 值)。