@Entity @Table注解

系统管理员 2022-05-13 01:20 181阅读 0赞

Java Persistence API中定义了两个注解@Entity与@Table

@Entity表明这是一个实体类，要与数据库做orm映射，默认表的名字就是类名，表中的字段就是类中的属性。它的定义如下：其中name属性表示用JPQL语句时写的表的名字，如果没有在@Table注解中指定表名，这个名字也将作为表名映射到数据库

@Documented
    @Target(TYPE)
    @Retention(RUNTIME)
    public @interface Entity {
    
    	/**
    	 * (Optional) The entity name. Defaults to the unqualified
    	 * name of the entity class. This name is used to refer to the
    	 * entity in queries. The name must not be a reserved literal
    	 * in the Java Persistence query language.
    	 */
    	String name() default "";
    }

@Table 就是改变某些默认的映射规则，如表名，schema等，可以添加索引和约束，看一下它的定义

@Target(TYPE)
    @Retention(RUNTIME)
    public @interface Table {
    	/**
    	 * (Optional) The name of the table.
    	 * <p/>
    	 * Defaults to the entity name.
    	 */
    	String name() default "";
    
    	/**
    	 * (Optional) The catalog of the table.
    	 * <p/>
    	 * Defaults to the default catalog.
    	 */
    	String catalog() default "";
    
    	/**
    	 * (Optional) The schema of the table.
    	 * <p/>
    	 * Defaults to the default schema for user.
    	 */
    	String schema() default "";
    
    	/**
    	 * (Optional) Unique constraints that are to be placed on
    	 * the table. These are only used if table generation is in
    	 * effect. These constraints apply in addition to any constraints
    	 * specified by the <code>Column</code> and <code>JoinColumn</code>
    	 * annotations and constraints entailed by primary key mappings.
    	 * <p/>
    	 * Defaults to no additional constraints.
    	 */
    	UniqueConstraint[] uniqueConstraints() default { };
    
    	/**
    	 * (Optional) Indexes for the table. These are only used if table generation is in effect.  Defaults to no
    	 * additional indexes.
    	 *
    	 * @return The indexes
    	 */
    	Index[] indexes() default {};
    }

name: 用来指定要映射的表名

catalog: 指定table数据库中的路径，这里指数据库名

schema: 映射到哪个schema，用户名

uniqueConstraints： 可以创建单个字段或者联合的唯一约束，只有在创建表成功时才执行

indexes：索引，可以指定联合索引，也是只有表生成成功才有效

我的实体类代码，（只为了演示注解效果，在表结构设计上是非常不规范的，没有实际意义）

package com.xhx.springboot.entity;
    
    import javax.persistence.*;
    
    /**
     * @author xuhaixing
     * @date 2018/4/28 10:29
     */
    @Entity(name = "Acc")
    @Table(name = "account", //表名称
            catalog = "mytest2",   //数据库名
            schema = "xuhaixing",  //用户名
            uniqueConstraints = {   //联合或者单个字段 唯一约束
            @UniqueConstraint(columnNames = {"name","money"}), @UniqueConstraint(columnNames = {"name"})
            },
            indexes = {
            @Index(columnList = "name,version")
            }
            )
    public class Account {
        @Id
        private int id;
        private String name;
        private Double money;
        @Version
        private int version;
    
    
        public int getVersion() {
            return version;
        }
    
        public void setVersion(int version) {
            this.version = version;
        }
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Double getMoney() {
            return money;
        }
    
        public void setMoney(Double money) {
            this.money = money;
        }
    }

程序启动过程中执行的sql语句：

Hibernate: create table mytest2.account (id integer not null, money double precision, name varchar(255), version integer not null, primary key (id)) engine=MyISAM
    Hibernate: create index IDXpdrwpyis5o4yn1f3n52oc756e on mytest2.account (name, version)
    Hibernate: alter table mytest2.account drop index UKdqfeeghd5jfgxqfxae9urq0em
    Hibernate: alter table mytest2.account add constraint UKdqfeeghd5jfgxqfxae9urq0em unique (name, money)
    Hibernate: alter table mytest2.account drop index UKbb9lrmwswqvhcy1y430ki00ir
    Hibernate: alter table mytest2.account add constraint UKbb9lrmwswqvhcy1y430ki00ir unique (name)

UniqueConstraint的源码如下，可以用name指定约束的名字

@Target({ })
    @Retention(RUNTIME)
    public @interface UniqueConstraint {
    	/**
    	 * (Optional) Constraint name.  A provider-chosen name will be chosen
    	 * if a name is not specified.
    	 *
    	 * @since Java Persistence 2.0
    	 */
    	String name() default "";
    
    	/**
    	 * (Required) An array of the column names that make up the constraint.
    	 */
    	String[] columnNames();
    }

Index注解源码如下，还可以用name指定索引的名字

@Target({}) @Retention(RUNTIME)
    public @interface Index {
    	/**
    	 * (Optional) The name of the index.  Defaults to a provider-generated value.
    	 *
    	 * @return The index name
    	 */
    	String name() default "";
    
    	/**
    	 * (Required) The names of the columns to be included in the index.
    	 *
    	 * @return The names of the columns making up the index
    	 */
    	String columnList();
    
    	/**
    	 * (Optional) Whether the index is unique.  Default is false.
    	 *
    	 * @return Is the index unique?
    	 */
    	boolean unique() default false;
    }

我的dao层代码：

package com.xhx.springboot.dao;
    
    import com.xhx.springboot.entity.Account;
    import org.springframework.data.jpa.repository.JpaRepository;
    import org.springframework.data.jpa.repository.Modifying;
    import org.springframework.data.jpa.repository.Query;
    import org.springframework.data.repository.CrudRepository;
    import org.springframework.data.repository.query.Param;
    import org.springframework.stereotype.Repository;
    
    /**
     * @author xuhaixing
     * @date 2018/5/2 11:19
     */
    @Repository
    public interface AccountDao extends JpaRepository<Account, Integer> {
    
    
        @Modifying
        @Query("update Acc set name=:name, money=:money where id=:id")
        int updateAccount(@Param("id") int id,@Param("name") String name, @Param("money") double money);
    
    
        @Modifying  //通知jpa这是一个update或者delete操作，jpql不支持insert操作
        @Query("update Acc set name=:name, money=:money,version=:version+1 where id=:id and version=:version")
        int updateAccountByVersion(@Param("id") int id,@Param("name") String name, @Param("money") double money,@Param("version") int version);
    
        @Query(value = "select count(*) from account",nativeQuery = true)//使用原生sql进行查询
        long selectCount();
    
    }

至此，@Table和@Entity的注解所有属性就讲解完了

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