java - 在 Controller 中注入(inject)构造函数时出错，java.lang.NullPointerException

Question

我收到以下错误消息:

ProvisionException: Unable to provision, see the following errors:

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

1 error

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;
    }
}

我不太确定我该怎么做。我一直在到处搜索，找不到任何有用的东西。想法？

Answer 1

我想通了，我需要将我的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，我需要一个带有 FormFactory 的构造函数来注入(inject):

private Form<Product> productForm;

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