JPA查找實體

要找到一個實體，EntityManger接口提供了find()方法，該方法根據主鍵搜索一個元素。

JPA實體查找示例

在這裏，我們將搜索指定的記錄並在控制檯輸出它的值。

完整的項目代碼如下所示 -

這個例子包含以下步驟 -

第1步: 在com.yiibai.jpa.student包下創建一個名爲StudentEntity.java的實體類，它包含屬性:s_id，s_name和s_age。

文件:StudentEntity.java 的代碼如下 -

package com.yiibai.jpa.student;

import javax.persistence.*;

@Entity
@Table(name = "student")
public class StudentEntity {

    @Id
    private int s_id;
    private String s_name;
    private int s_age;

    public StudentEntity(int s_id, String s_name, int s_age) {
        super();
        this.s_id = s_id;
        this.s_name = s_name;
        this.s_age = s_age;
    }

    public StudentEntity() {
        super();
    }

    public int getS_id() {
        return s_id;
    }

    public void setS_id(int s_id) {
        this.s_id = s_id;
    }

    public String getS_name() {
        return s_name;
    }

    public void setS_name(String s_name) {
        this.s_name = s_name;
    }

    public int getS_age() {
        return s_age;
    }

    public void setS_age(int s_age) {
        this.s_age = s_age;
    }

}

第2步: 將實體類和其他數據庫配置映射到persistence.xml文件中。

文件:persistence.xml 的代碼如下 -

<?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="Student_details">
        <class>com.yiibai.jpa.student.StudentEntity</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
                value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                value="jdbc:mysql://localhost:3306/testdb?serverTimezone=UTC" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="123456" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation"
                value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

在com.yiibai.jpa.student包下創建一個名爲FindStudent.java的持久化類，以便將實體對象與數據保持一致。

文件:FindStudent.java 的代碼如下 -

package com.yiibai.jpa.student;

import javax.persistence.*;

import com.yiibai.jpa.student.*;

public class FindStudent {
    public static void main(String args[]) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("Student_details");
        EntityManager em = emf.createEntityManager();

        StudentEntity s = em.find(StudentEntity.class, 1001);

        System.out.println("Student id = " + s.getS_id());
        System.out.println("Student Name = " + s.getS_name());
        System.out.println("Student Age = " + s.getS_age());

    }
}

執行上面示例代碼，得到以下結果 -

Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
Fri Jun 08 14:09:45 CST 2018 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Fri Jun 08 14:09:45 CST 2018 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Student id = 1001
Student Name = Maxsu
Student Age = 26