我在我的Bean中注入的EntityManager得到Nullpointer异常。我已经搜索了解决方案，但无法在帖子中找到任何内容，这帮助了我。 此外，我不明白为什么错误消息说，ExampleServlet中出现错误，我在我的应用程序中没有。
我的DAO Bean由一个方法调用，该方法由一个在PersonServlet中注册的odata处理程序调用：
public void initData() {
PersonDAO psDAO = new PersonDAO();
psDAO.getAllPersons();
}
我的情况可能是错误的原因是什么？
错误：
2017-01-11T18:46:58.721+0000|Severe: [http-listener-1(5)] ERROR test.personservice.web.PersonServlet - Server Error occurred in ExampleServlet
2017-01-11T18:46:58.722+0000|Severe: java.lang.NullPointerException
at test.personservice.dao.PersonDAO.getAllPersons(PersonDAO.java:21)
at test.personservice.data.Storage.initSampleData(Storage.java:94)
at test.personservice.data.Storage.<init>(Storage.java:30)
at test.personservice.web.PersonServlet.service(PersonServlet.java:35)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:416)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
at java.lang.Thread.run(Thread.java:745)
2017-01-11T18:46:58.723+0000|Warning: StandardWrapperValve[PersonServlet]: Servlet.service() for servlet PersonServlet threw exception
java.lang.NullPointerException
at test.personservice.dao.PersonDAO.getAllPersons(PersonDAO.java:21)
at test.personservice.data.Storage.initSampleData(Storage.java:94)
at test.personservice.data.Storage.<init>(Storage.java:30)
at test.personservice.web.PersonServlet.service(PersonServlet.java:35)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:416)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
at java.lang.Thread.run(Thread.java:745)
豆
@Stateless
public class PersonDAO {
@PersistenceContext
private EntityManager em;
public List<Person> getAllPersons() {
return
em.createQuery("SELECT p FROM T_Person p", Person.class).getResultList();
}
}
的persistence.xml
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="PersonService" transaction-type="JTA">
<jta-data-source>jdbc/PersonDB</jta-data-source>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
<property name="hibernate.hbm2ddl.auto" value="create"/>
<property name="hibernate.show_sql" value="yes"/>
</properties>
</persistence-unit>
</persistence>
的pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>test</groupId>
<artifactId>PersonService</artifactId>
<packaging>war</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>PersonService Maven Webapp</name>
<url>http://maven.apache.org</url>
<properties>
<!-- <javax.version>3.1</javax.version> -->
<odata.version>4.0.0</odata.version>
<slf4j.version>1.7.7</slf4j.version>
</properties>
<dependencies>
<!-- <dependency> -->
<!-- <groupId>javax.servlet</groupId> -->
<!-- <artifactId>servlet-api</artifactId> -->
<!-- <version>${javax.version}</version> -->
<!-- <scope>provided</scope> -->
<!-- </dependency> -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version>
</dependency>
<dependency>
<groupId>javax.ejb</groupId>
<artifactId>javax.ejb-api</artifactId>
<version>3.2</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.2.5.Final</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-validator</artifactId>
<version>5.3.4.Final</version>
</dependency>
<dependency>
<groupId>org.apache.olingo</groupId>
<artifactId>odata-server-api</artifactId>
<version>${odata.version}</version>
</dependency>
<dependency>
<groupId>org.apache.olingo</groupId>
<artifactId>odata-server-core</artifactId>
<version>${odata.version}</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>org.apache.olingo</groupId>
<artifactId>odata-commons-api</artifactId>
<version>${odata.version}</version>
</dependency>
<dependency>
<groupId>org.apache.olingo</groupId>
<artifactId>odata-commons-core</artifactId>
<version>${odata.version}</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
<version>${slf4j.version}</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.11</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<finalName>PersonService</finalName>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</plugin>
</plugins>
</build>
</project>
PersonServlet
public class PersonServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static final Logger LOG = LoggerFactory.getLogger(PersonServlet.class);
protected void service(final HttpServletRequest req, final HttpServletResponse resp) throws ServletException, IOException {
try {
HttpSession session = req.getSession(true);
Storage storage = (Storage) session.getAttribute(Storage.class.getName());
if (storage == null) {
storage = new Storage();
session.setAttribute(Storage.class.getName(), storage);
}
// create odata handler and configure it with CsdlEdmProvider and Processor
OData odata = OData.newInstance();
ServiceMetadata edm = odata.createServiceMetadata(new PersonEdmProvider(), new ArrayList<EdmxReference>());
ODataHttpHandler handler = odata.createHandler(edm);
handler.register(new PersonEntityCollectionProcessor(storage));
handler.register(new PersonEntityProcessor(storage));
handler.register(new PersonPrimitiveProcessor(storage));
// let the handler do the work
handler.process(req, resp);
} catch (RuntimeException e) {
LOG.error("Server Error occurred in ExampleServlet", e);
throw new ServletException(e);
}
}
}
存储
@Stateless
public class Storage {
@EJB
PersonDAO psDAO;
private List<Entity> personList;
public Storage() {
personList = new ArrayList<Entity>();
}
@PostConstruct
private void initSampleData(){
psDAO.getAllPersons();
}
}
的web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="3.1"
xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
<servlet>
<servlet-name>PersonServlet</servlet-name>
<servlet-class>test.personservice.web.PersonServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>PersonServlet</servlet-name>
<url-pattern>/PersonService.svc/*</url-pattern>
</servlet-mapping>
</web-app>
答案 0 :(得分：0)
您使用Storage自行创建new实例，因此Storage对象不受容器管理，因此无法在其中注入DAO。通过转换，由于DAO不为null，我可以安全地猜测Storage对象也使用new创建DAO，因此DAO不再由容器管理，因此可以＆＃ 39; t将实体经理注入其中。
使用依赖注入。不要自己创建存储和DAO。