18.4 应用@Autowired和@Service进行依赖注入

使用Spring框架的一个好处是容易进行依赖注入。毕竟，Spring框架一开始就是一个依赖注入容器。将依赖注入到Spring MVC控制器的最简单方法是通过注解@Autowired到字段或方法。Autowired注解类型属于org.springframework.beans.factory.annotation包。
此外，**为了能被作为依赖注入，类必须要注明为@Service**。该类型是org.springframework.stereotype包的成员。Service注解类型指示类是一个服务。此外，在配置文件中，还需要添加一个<component-scan/>元素来扫描依赖基本包：

<context:component-scan base-package="dependencyPackage"/>

下面以app18b应用进一步说明Spring MVC如何应用依赖注入。在app18b应用程序中，ProductController类已经不同于app18a。

app18b的ProductController类

package app18b.controller;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.servlet.mvc.support.RedirectAttributes;
import app18b.domain.Product;
import app18b.form.ProductForm;
import app18b.service.ProductService;
@Controller
public class ProductController
{
    private static final Log logger = LogFactory
        .getLog(ProductController.class);
    @Autowired
    private ProductService productService;
    @RequestMapping(value = "/product_input")
    public String inputProduct()
    {
        logger.info("inputProduct called");
        return "ProductForm";
    }
    @RequestMapping(
        value = "/product_save",
        method = RequestMethod.POST
    )
    public String saveProduct(ProductForm productForm,
        RedirectAttributes redirectAttributes)
    {
        logger.info("saveProduct called");
        // no need to create and instantiate a ProductForm
        // create Product
        Product product = new Product();
        product.setName(productForm.getName());
        product.setDescription(productForm.getDescription());
        try
        {
            product
                .setPrice(Float.parseFloat(productForm.getPrice()));
        } catch (NumberFormatException e)
        {}
        // add product
        Product savedProduct = productService.add(product);
        redirectAttributes.addFlashAttribute("message",
            "The product was successfully added.");
        return "redirect:/product_view/" + savedProduct.getId();
    }
    @RequestMapping(value = "/product_view/{id}")
    public String viewProduct(@PathVariable
    Long id,Model model)
    {
        Product product = productService.get(id);
        model.addAttribute("product", product);
        return "ProductView";
    }
}

与app18a中相比，app18b中的ProductController类做了一系列的调整。首先是在如下的私有字段上增加了@Autowired注解：

@Autowired
private ProductService productService

ProductService是一个提供各种处理产品方法的接口。为productService字段添加@Autowired注解会使ProductService的一个实例被注入到ProductController实例中。
下面代码分别显示了ProductService接口及其实现类ProductServiceImpl。注意，**为了使类能被Spring扫描到，必须为其标注@Service**。

ProductService接口

package app18b.service;
import app18b.domain.Product;
public interface ProductService
{
    Product add(Product product);
    Product get(long id);
}

ProductServiceImpl类

package app18b.service;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicLong;
import org.springframework.stereotype.Service;
import app18b.domain.Product;
@Service
public class ProductServiceImpl
    implements
        ProductService
{
    private Map<Long,Product> products = new HashMap<Long,Product>();
    private AtomicLong generator = new AtomicLong();
    public ProductServiceImpl()
    {
        Product product = new Product();
        product.setName("JX1 Power Drill");
        product.setDescription(
            "Powerful hand drill, made to perfection");
        product.setPrice(129.99F);
        add(product);
    }
    @Override
    public Product add(Product product)
    {
        long newId = generator.incrementAndGet();
        product.setId(newId);
        products.put(newId, product);
        return product;
    }
    @Override
    public Product get(long id)
    {
        return products.get(id);
    }
}

Spring MVC配置文件

app18b的Spring MVC配置文件中有两个<component-scan/>元素；一个用于扫描控制器类，另一个用于扫描服务类。

SpringMVC配置文件

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc.xsd     
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">
    <!-- 扫描控制器 -->
    <context:component-scan
        base-package="app18b.controller" />
    <!-- 扫描服务 -->
    <context:component-scan
        base-package="app18b.service" />
    <mvc:annotation-driven />
    <mvc:resources mapping="/css/**" location="/css/" />
    <mvc:resources mapping="/*.html" location="/" />
    <!-- 视图解析器 -->
    <bean id="viewResolver"
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/jsp/" />
        <property name="suffix" value=".jsp" />
    </bean>
</beans>