Question

我通过采用一个发现的例子来学习Spring框架。该示例如下实例化一个EntityManager对象：

package com.example.demo;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Objects;
import java.util.Optional;
import java.util.function.Consumer;

import javax.persistence.EntityManager;
import javax.persistence.EntityTransaction;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import jpautils.JPAUtils;
import utils.StringUtil;

/**
 * @author 
 *
 */
public class JPAUserDAO implements IDAO<User> {

@PersistenceContext
private final EntityManager entityManager;

/**
 * Constructor
 */
public JPAUserDAO() {
    entityManager = JPAUtils.getEntityManager();
}

@Override
public void delete(final User user) {
    executeInsideTransaction(entityManager -> entityManager.remove(user));
}

private void executeInsideTransaction(final Consumer<EntityManager> action) {
    final EntityTransaction tx = entityManager.getTransaction();

    try {
        tx.begin();

        action.accept(entityManager);

        tx.commit();
    } catch (final RuntimeException e) {
        tx.rollback();
        throw e;
    }
}
[…]

不幸的是，某些错误的设置会导致运行时错误：

UserApplication.main([2])
17:28:29.500 [main] DEBUG org.jboss.logging - Logging Provider: org.jboss.logging.Log4j2LoggerProvider
17:28:29.526 [main] INFO org.hibernate.jpa.boot.internal.PersistenceXmlParser - HHH000318: Could not find any META-INF/persistence.xml file in the classpath
17:28:29.526 [main] DEBUG org.hibernate.jpa.HibernatePersistenceProvider - Located and parsed 0 persistence units; checking each
17:28:29.526 [main] DEBUG org.hibernate.jpa.HibernatePersistenceProvider - Found no matching persistence units
javax.persistence.PersistenceException: No Persistence provider for EntityManager named com.example.demo
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:61)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:39)
at jpautils.JPAUtils.<clinit>(JPAUtils.java:16)
at com.example.demo.JPAUserDAO.<init>(JPAUserDAO.java:35)
at com.example.demo.UserApplication.main(UserApplication.java:30)
Exception in thread "main" java.lang.NullPointerException
at jpautils.JPAUtils.getEntityManager(JPAUtils.java:23)
at com.example.demo.JPAUserDAO.<init>(JPAUserDAO.java:35)
at com.example.demo.UserApplication.main(UserApplication.java:30)

我阅读了类似的帖子，例如this和that，但仍然无法解决我的问题。也许，解决此问题的一个线索可能是在包含module-info.java文件时出现的错误：

/**
 *
 */
/**
 * The module for the Web site "Shop".
 *
 * @author 
 *
 */
module WSShop {
    exports com.example.demo;

    requires java.persistence;
    requires java.sql;
    requires spring.boot;
    requires spring.boot.autoconfigure;
    requires Utilities;
}

导致以下错误；对此文件添加注释会删除所有编译时错误：

Description Resource    Path    Location    Type
Syntax error on token ".", , expected   module-info.java    /WSShop/src/main/java   line 13 Java Problem
Syntax error on token ".", , expected   module-info.java    /WSShop/src/main/java   line 14 Java Problem
Syntax error on token ".", , expected   module-info.java    /WSShop/src/main/java   line 15 Java Problem
Syntax error on token ".", = expected   module-info.java    /WSShop/src/main/java   line 11 Java Problem
Syntax error on token ".", = expected   module-info.java    /WSShop/src/main/java   line 16 Java Problem
Syntax error on token "module", interface expected  module-info.java    /WSShop/src/main/java   line 10 Java Problem

module-info.java有什么问题？我的persistence.xml中缺少或存在什么问题？

这也是pom.xml文件：

<?xml version="1.0" encoding="UTF-8"?>
<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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<!-- parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.6.RELEASE</version>
    <relativePath /> <!- lookup parent from repository ->
</parent -->
<groupId>com.example</groupId>
<artifactId>WSShop</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>WSShop</name>
<description>Demo project for Spring Boot</description>

<properties>
    <java.version>12</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>5.3.10.Final</version>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.14</version>
    </dependency>
    <dependency>
        <groupId>org.hibernate.javax.persistence</groupId>
        <artifactId>hibernate-jpa-2.1-api</artifactId>
        <version>1.0.2.Final</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>2.1.6.RELEASE</version>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
        <version>2.1.6.RELEASE</version>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>
</project>

编辑： 这里有两个错误信息：

在春季示例中无法创建EntityManager对象

0 个答案: