使用 spring boot 和 spring 数据 JPA 的 CriteriaQuery 中的空指针异常

Question

我正在尝试使用 CriteriaQuery 和 spring boot 和 spring 数据 JPA 来实现 Criteria API 的一个示例。 当我运行项目时，我收到错误空指针异常，

错误，

java.lang.NullPointerException: null
at com.spacestudy.controller.AuthenticationController.load(AuthenticationController.java:57) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]

来自我的控制器的测试代码是，

@GetMapping("/load")
    public void load() {

        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Permissions> cq = cb.createQuery(Permissions.class);
        Root<Permissions> rPermission = cq.from(Permissions.class);
        cq.select(rPermission);
        cq.where(cb.equal(rPermission.get("nuserId"),730));    
        List<Permissions> results = em.createQuery(cq).getResultList();
        for (Permissions permission : results) {
            System.out.println("Number = " + permission.getSpermissionType());
        }
    }

堆栈跟踪，

Java.lang.NullPointerException: null
at com.spacestudy.controller.AuthenticationController.load(AuthenticationController.java:57) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:877) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:783) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.31.jar:8.5.31]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851) ~[spring-webmvc-5.0.7.RELEASE.jar:5.0.7.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.31.jar:8.5.31]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.31.jar:8.5.31]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.31.jar:8.5.31]

我是这个 Criteria API 的新手。 任何人都可以帮我解决这个问题吗？

Answer 1

EntityManager 应该是@PersistenceContext。

@PersistenceContext – 我们需要了解如何使用简单的注解@PersistenceContext 和它是什么来连接数据库。

 @PersistenceContext
 private EntityManager emf;

 @GetMapping("/load")
 public void load() {
    CriteriaBuilder cb = emf.getCriteriaBuilder();
    CriteriaQuery<Permissions> cq = cb.createQuery(Permissions.class);
    Root<Permissions> rPermission = cq.from(Permissions.class);
    cq.select(rPermission);
    cq.where(cb.equal(rPermission.get("nuserId"),730));    
    List<Permissions> results = em.createQuery(cq).getResultList();
    for (Permissions permission : results) {
        System.out.println("Number = " + permission.getSpermissionType());
    }
}