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一旦我們將實體保存到數(shù)據(jù)庫中,我們可以通過使用EntityManager中的find方法來檢索它們。
以下代碼顯示如何使用find方法與實體id。
Person emp = em.find(Person.class, 1L);
下面的代碼來自Person.java。
package cn.w3cschool.common; import static javax.persistence.FetchType.LAZY; import javax.persistence.Basic; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.Lob; import javax.persistence.Table; @Entity @Table(name="EMP") public class Person { @Id @Column(name = "EMP_ID") private long id; @Basic private String name; private String surname; @Lob @Basic(fetch=LAZY) private byte[] picture; public Person() {} public Person(String name, String surname) { this.name = name; this.surname = surname; } public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getSurname() { return surname; } public void setSurname(String surname) { this.surname = surname; } public byte[] getPicture() { return picture; } public void setPicture(byte[] picture) { this.picture = picture; } @Override public String toString() { return "Person [id=" + id + ", name=" + name + ", surname=" + surname + "]"; } }
下面的代碼來自PersonDaoImpl.java。
package cn.w3cschool.common; import java.util.List; import javax.persistence.EntityManager; import javax.persistence.PersistenceContext; import org.springframework.transaction.annotation.Transactional; @Transactional public class PersonDaoImpl { public void test(){ Person p1 = new Person("Tom", "Smith"); p1.setId(1L); p1.setPicture("asdf".getBytes()); Person p2 = new Person("Jack", "Kook"); p2.setId(2L); p1.setPicture("m.o2fo.com".getBytes()); save(p1); save(p2); listAll(); Person emp = em.find(Person.class, 1L); if (emp != null) { em.remove(emp); } listAll(); } private void listAll(){ List<Person> persons = getAll(); for (Person person : persons) { System.out.println(person); } } @PersistenceContext private EntityManager em; public Long save(Person person) { em.persist(person); return person.getId(); } public List<Person>getAll() { return em.createQuery("SELECT p FROM Person p", Person.class).getResultList(); } }下載 Find_By_ID.zip
上面的代碼生成以下結(jié)果。
以下是數(shù)據(jù)庫轉(zhuǎn)儲。
Table Name: EMP Row: Column Name: EMP_ID, Column Type: BIGINT: Column Value: 2 Column Name: NAME, Column Type: VARCHAR: Column Value: Jack Column Name: PICTURE, Column Type: BLOB: Column Value: null Column Name: SURNAME, Column Type: VARCHAR: Column Value: Kook
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