在控制器中注入构造函数时出错，java.lang.NullPointerException

Question

我收到以下错误消息：

ProvisionException：无法配置，请参阅以下错误：

1）在controllers.ProductsController.(ProductsController.java:20)中注入构造函数时出错，
在为router.Routes.(Routes.scala:34)的第四个参数定位controllers.ProductsController时在controllers.ProductsController.(Routes.scala:34)中定位router.Routes时出错定位 play.api.inject.RoutesProvider，同时定位 play.api.routing.Router 作为 play.api.http.JavaCompatibleHttpRequestHandler.(HttpRequestHandler.scala:222) 的第一个参数，同时在定位 play.api.http.JavaCompatibleHttpRequestHandler 时定位 play.api.http.JavaCompatibleHttpRequestHandler。 api.http.HttpRequestHandler 用于 play.api.DefaultApplication.(Application.scala:236) 在 play.api.DefaultApplication.class(Application.scala:235) 的第 6 个参数，同时定位 play.api.DefaultApplication 同时定位 play.api 。应用

1 个错误

这是堆栈跟踪：

com.google.inject.ProvisionException: Unable to provision, see the following errors:

1) Error injecting constructor, java.lang.NullPointerException
  at controllers.ProductsController.<init>(ProductsController.java:20)
  while locating controllers.ProductsController
    for the 4th parameter of router.Routes.<init>(Routes.scala:34)
  while locating router.Routes
  while locating play.api.inject.RoutesProvider
  while locating play.api.routing.Router
    for the 1st parameter of play.api.http.JavaCompatibleHttpRequestHandler.<init>(HttpRequestHandler.scala:222)
  while locating play.api.http.JavaCompatibleHttpRequestHandler
  while locating play.api.http.HttpRequestHandler
    for the 6th parameter of play.api.DefaultApplication.<init>(Application.scala:236)
  at play.api.DefaultApplication.class(Application.scala:235)
  while locating play.api.DefaultApplication
  while locating play.api.Application

1 error
     com.google.inject.internal.InjectorImpl$2.get(InjectorImpl.java:1028)
     com.google.inject.internal.InjectorImpl.getInstance(InjectorImpl.java:1054)
     play.api.inject.guice.GuiceInjector.instanceOf(GuiceInjectorBuilder.scala:409)
     play.api.inject.guice.GuiceInjector.instanceOf(GuiceInjectorBuilder.scala:404)

这是我的代码：

package controllers;

import java.util.List;

import javax.inject.Inject;
import javax.inject.Provider;
import javax.inject.Singleton;

import play.mvc.Controller;
import play.mvc.Result;

import play.data.Form;
import play.data.FormFactory;

import views.html.products.details;
import views.html.products.list;

import models.Product;

public class ProductsController extends Controller {

    @Inject FormFactory formFactory;
    private Form<Product> productForm = formFactory.form(Product.class);

    public Result list() {
        List<Product> products = Product.findAll();
        return ok(list.render(products));
    }

    public Result newProduct() {
        return ok(details.render(productForm));
    }

    public Result details(String ean) {
        return TODO;
    }

    public Result save() {
        return TODO;
    }
}

我不太确定我应该做什么。我到处寻找，但找不到任何有用的东西。想法？

Answer 1

我发现了这一点，我需要将代码更改为如下所示：

package controllers;

import java.util.List;

import javax.inject.Inject;
import javax.inject.Provider;
import javax.inject.Singleton;

import play.mvc.Controller;
import play.mvc.Result;

import play.data.Form;
import play.data.FormFactory;

import views.html.products.details;
import views.html.products.list;

import models.Product;

public class ProductsController extends Controller {

    private Form<Product> productForm;

    @Inject
    public ProductsController(FormFactory formFactory) {
        this.productForm = formFactory.form(Product.class);
    }

    public Result list() {
        List<Product> products = Product.findAll();
        return ok(list.render(products));
    }

    public Result newProduct() {
        return ok(details.render(productForm));
    }

    public Result details(String ean) {
        return TODO;
    }

    public Result save() {
        return TODO;
    }
}

具体来说，我需要有一个FormFactory要注入的构造函数：

private Form<Product> productForm;

@Inject
public ProductsController(FormFactory formFactory) {
    this.productForm = formFactory.form(Product.class);
}