注意:这是一个常见问题的规范答案。
我有一个Spring @Service类(MileageFeeCalculator),它有一个@Autowired字段(rateService),但当我尝试使用它时,该字段为空。日志显示正在创建MileageFeeCalculator bean和MileageRateService bean,但是每当我试图在服务bean上调用mileageCharge方法时,都会得到一个NullPointerException。为什么Spring没有自动装配该字段?
控制器类:
@Controller
public class MileageFeeController {
@RequestMapping("/mileage/{miles}")
@ResponseBody
public float mileageFee(@PathVariable int miles) {
MileageFeeCalculator calc = new MileageFeeCalculator();
return calc.mileageCharge(miles);
}
}
服务类:
@Service
public class MileageFeeCalculator {
@Autowired
private MileageRateService rateService; // <--- should be autowired, is null
public float mileageCharge(final int miles) {
return (miles * rateService.ratePerMile()); // <--- throws NPE
}
}
服务bean,应该在MileageFeeCalculator自动连接,但它不是:
@Service
public class MileageRateService {
public float ratePerMile() {
return 0.565f;
}
}
当我尝试GET /mileage/3时,我得到了这个异常:
java.lang.NullPointerException: null
at com.chrylis.example.spring_autowired_npe.MileageFeeCalculator.mileageCharge(MileageFeeCalculator.java:13)
at com.chrylis.example.spring_autowired_npe.MileageFeeController.mileageFee(MileageFeeController.java:14)
...
我是Spring的新手,但我发现了这个可行的解决方案。请告诉我这是一种不可取的方式。
我让Spring在这个bean中注入applicationContext:
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;
@Component
public class SpringUtils {
public static ApplicationContext ctx;
/**
* Make Spring inject the application context
* and save it on a static variable,
* so that it can be accessed from any point in the application.
*/
@Autowired
private void setApplicationContext(ApplicationContext applicationContext) {
ctx = applicationContext;
}
}
如果愿意,也可以将此代码放在主应用程序类中。
其他类可以这样使用它:
MyBean myBean = (MyBean)SpringUtils.ctx.getBean(MyBean.class);
通过这种方式,应用程序中的任何对象(也用new实例化)都可以以静态方式获取任何bean。
你的问题是新的(java风格的对象创建)
MileageFeeCalculator calc = new MileageFeeCalculator();
使用注释@Service、@Component、@Configuration bean在
当服务器启动时,Spring的应用程序上下文。但是当我们创建对象时
使用new操作符,对象不会在已经创建的应用程序上下文中注册。例如Employee.java类,我使用过。
看看这个:
public class ConfiguredTenantScopedBeanProcessor implements BeanFactoryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
String name = "tenant";
System.out.println("Bean factory post processor is initialized");
beanFactory.registerScope("employee", new Employee());
Assert.state(beanFactory instanceof BeanDefinitionRegistry,
"BeanFactory was not a BeanDefinitionRegistry, so CustomScope cannot be used.");
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
for (String beanName : beanFactory.getBeanDefinitionNames()) {
BeanDefinition definition = beanFactory.getBeanDefinition(beanName);
if (name.equals(definition.getScope())) {
BeanDefinitionHolder proxyHolder = ScopedProxyUtils.createScopedProxy(new BeanDefinitionHolder(definition, beanName), registry, true);
registry.registerBeanDefinition(beanName, proxyHolder.getBeanDefinition());
}
}
}
}