所以...我刚刚开始使用数据库,而我的兄弟建议我将H2与EclipseLink一起使用来启动。在Google上进行了快速搜索并找到了Tutuorial:
https://www.javatips.net/blog/eclipselink-jpa-with-h2-database
https://www.javatips.net/blog/java-persistence-jpa-2-0-tutorial-with-eclipselink
如果我按照本教程中的描述进行编码,则在运行它时不会出错,但是我检查数据库时我的表仍然为空。我现在搜索了大约一个星期,但找不到答案。
我的代码:
班级学生
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name = "STUDENT")
public class Student implements java.io.Serializable {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "STUDENTID")
private long studentId;
@Column(name = "STUDENTNAME")
private String studentName;
public void setStudentId(long studentId) {
this.studentId = studentId;
}
public long getStudentId() {
return studentId;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
public String getStudentName() {
return studentName;
}
}
JPAExample
import java.util.Iterator;
import java.util.List;
import javax.persistence.EntityManager;
public class JPAExample {
private EntityManager entityManager = EntityManagerUtil.getEntityManager();
public static void main(String[] args) {
JPAExample example = new JPAExample();
System.out.println("After Sucessfully insertion ");
Student student1 = example.saveStudent("Sumith");
Student student2 = example.saveStudent("Anoop");
example.listStudent();
System.out.println("After Sucessfully modification ");
example.updateStudent(student1.getStudentId(), "Sumith Honai");
example.updateStudent(student2.getStudentId(), "Anoop Pavanai");
example.listStudent();
System.out.println("After Sucessfully deletion ");
example.deleteStudent(student2.getStudentId());
example.listStudent();
}
public Student saveStudent(String studentName) {
Student student = new Student();
try {
entityManager.getTransaction().begin();
student.setStudentName(studentName);
student = entityManager.merge(student);
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
}
return student;
}
public void listStudent() {
try {
entityManager.getTransaction().begin();
@SuppressWarnings("unchecked")
List<Student> Students = entityManager.createQuery("from Student").getResultList();
for (Iterator<Student> iterator = Students.iterator(); iterator.hasNext();) {
Student student = (Student) iterator.next();
System.out.println(student.getStudentName());
}
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
}
}
public void updateStudent(Long studentId, String studentName) {
try {
entityManager.getTransaction().begin();
Student student = (Student) entityManager.find(Student.class, studentId);
student.setStudentName(studentName);
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
}
}
public void deleteStudent(Long studentId) {
try {
entityManager.getTransaction().begin();
Student student = (Student) entityManager.find(Student.class, studentId);
entityManager.remove(student);
entityManager.getTransaction().commit();
} catch (Exception e) {
entityManager.getTransaction().rollback();
}
}
}
EntityManagerUtil
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public class EntityManagerUtil {
private static final EntityManagerFactory entityManagerFactory;
static {
try {
entityManagerFactory = Persistence.createEntityManagerFactory("test");
} catch (Throwable ex) {
System.err.println("Initial SessionFactory creation failed." + ex);
throw new ExceptionInInitializerError(ex);
}
}
public static EntityManager getEntityManager() {
return entityManagerFactory.createEntityManager();
}
}
Persistence.xml
<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
<persistence-unit name="test" transaction-type="RESOURCE_LOCAL">
<class>Student</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="org.h2.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:h2:~/test" />
<property name="javax.persistence.jdbc.user" value="sa" />
<property name="javax.persistence.jdbc.password" value="" />
<property name="eclipselink.ddl-generation" value="create-tables"/>
<property name="eclipselink.ddl-generation.output-mode" value="database" />
</properties>
</persistence-unit>
当我运行JPAExample时,我得到以下输出: My Output
但是预期的输出应如下所示: Expected Output
如果我查看H2数据库,则什么都没有改变: H2 Database Interface (German Language)
希望有人可以帮助我,或者可以为我提供有用/更好的JPA教程:)
答案 0 :(得分:0)
您正在捕获异常,但未确认或记录它们-您应确保至少记录下来。
在这种情况下,您的listStudent方法在
上出现异常 entityManager.createQuery("from Student").getResultList()
,因为这是无效的JPQL查询。您应该使用类似这样的东西:
entityManager.createQuery("select s from Student s").getResultList()