SQLite数据库和JPA简单介绍

SQLite数据库和JPA简单介绍

 

SQLite 数据库和JPA简单介绍 SQLite数据库和JPA简单介绍

 

 

一、SQLite简单使用

SQLite是遵循ACID的关系数据库管理系统，它的处理速度很快，它的设计目标是嵌入式的，只需要几百K的内存就可以了。

 

1.下载SQLitehttp://www.sqlite.org/download.htmlsqlite-dll-win32-x86-201410071659.zip(294.11 KiB)sqlite-shell-win32-x86-3080600.zip解压在文件夹D:\Database\sqlite下，得到文件sqlite3.dll，sqlite3.exe参考：http://www.w3cschool.cc/sqlite/sqlite-installation.html

 

2.安装SQLite写批处理文件z_sqlite.bat，内容是：

@echod:cd D:\Database\sqlitecmd.exe

执行命令sqlite3，看到结果：

D:\Database\sqlite>sqlite3SQLite version 3.8.6 2014-08-15 11:46:33Enter ".help" for usage hints.Connected to a transient in-memory database.Use ".open FILENAME" to reopen on a persistent database.sqlite>

或是：

@echoPATH=%PATH%;D:\Database\sqlitecmd.exe

执行命令sqlite3，看到结果：

ECHO 处于打开状态。D:\Database>PATH=C:\PRogramData\Oracle\java\javapath;D:\Program_Files\oraclexe\app\oracle\product\10.2.0\server\bin;C:\Program Files\AMD APP\bin\x86;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\ATI Technologies\ATI.ACE\Core-Static;C:\Python27;C:\Program Files\MySQL\MySQL Server 5.5\bin;D:\Idea\config\apache-maven-3.2.3\bin;D:\Idea\config\apache-ant-1.9.4\bin;D:\Database\sqliteD:\Database>cmd.exeMicrosoft Windows [版本 6.3.9600](c) 2013 Microsoft Corporation。保留所有权利。D:\Database>sqlite3SQLite version 3.8.6 2014-08-15 11:46:33Enter ".help" for usage hints.Connected to a transient in-memory database.Use ".open FILENAME" to reopen on a persistent database.sqlite>

表明sqlite已安装。解释：不让bat文件运行命令结束后cmd窗口自动关闭，最后运行一下cmd.exe

更多疑问请参考：[SQLite 教程]：http://www.w3cschool.cc/sqlite/sqlite-installation.html

 

3.SQLite命令

.help　　　　帮助.show　　　显示各种设置的当前值.schema　　 显示建表语句

sqlite> .schema sqlite_mastercreate table episodes( id integer primary key, season int, name text );

其它的命令类似于一搬的sql语句

insert into episodes values(1, 1, 'one');select * from episodes;

 

4.SQLite管理工具：SQLiteExpert下载：http://www.sqliteexpert.com/download.html选择免费版：SQLite Expert Personal下载的太慢了，可以选择以前的版本：http://dl.dbank.com/c0sog1u0xb

创建db文件：

D:\Database\sqlite>sqlite3 test1.dbSQLite version 3.8.6 2014-08-15 11:46:33Enter ".help" for usage hints.sqlite> sqlite3 test1.db;Error: near "sqlite3": syntax errorsqlite> 

看到在D:\Database\sqlite目录下生成了一个test1.db的文件File>OpenDatabase>选择刚刚生成的db文件>确定看到数据库的文件已经导入：

二、JPA介绍Java Persistence API：简化现有的持久化开发工作和整合ORM技术结束现在的Hibernate、TopLink、JDO等ORM框架各自为营的局面面向Hibernate的API开发，会紧密的与Hibernate耦合JPA规范是由Hibernate的作者制定的

ORM映射元数据：　　描述对象与表之间的映射关系Java持久化API：　　 执行CRUD操作查询语言：　　　　 JPQL，避免程序和SQL语句的紧密耦合

设置Maven4MyEclipse：MyEclipse>Maven4MyEclipse

Installation=D:\Idea\config\apache-maven-3.2.3User Settings=D:\Idea\config\apache-maven-3.2.3\conf\settings.xmlLocal Repository=D:\Idea\maven\repository

配置文件：META-INF/persistence.xmlD:\Idea\config\hibernate-release-4.3.6.Final\lib\jpa-metamodel-generator/hibernate-jpamodelgen-4.3.6.Final.jar/persistence_2_1.xsd配置数据库的方言，以及自动建表

 

三、SQLite作为Hibernate的数据源1.Maven配置文件：pom.xml用MyEclipse新建一个Maven的模块app1.module1配置它的pom文件：/module1/pom.xml

引入JPA的依赖：http://hibernate.org/orm/downloads/

　　org.hibernate　　hibernate-entitymanager　　4.3.6.Final

sqlite-jdbc引入sqlite的依赖：http://www.mvnrepository.com/artifact/org.xerial/sqlite-jdbc/3.7.2

　　org.xerial　　sqlite-jdbc　　3.7.2

处理报错：ArtifactTransferException: Failure to transfer org.xerial:sqlite-jdbc:jar:3.7.2 from http://zhanghua.1199.blog.163.com/blog/static/464498072013529936189/

对于这个包从maven中心传输到本地仓库失败，决定不会重新尝试下载jar包，直到mavne再改更新索引，或强制更新。实际的解决办法是：直接去本地仓库，把这个1.1.1的目录删除掉（因为包没有下载下来），再次刷新你的项目就中以了，或者在你的项目上右击，选择maven--->update就可以了，让maven重新下载。

删除D:\Idea\maven\repository\org\xerial文件

/module1/pom.xml

    4.0.0    yuki.jpa.hibernate.app1    module1    0.0.1-SNAPSHOT    jar    module1    http://maven.apache.org            UTF-8                            junit            junit            4.11            test                                    org.hibernate            hibernate-entitymanager            4.3.6.Final                            org.xerial            sqlite-jdbc            3.7.2            

 

2.JPA的配置文件：persistence.xmlD:\Workspaces\IntelliJ_IDEA\JavaEE_JPA\Module1\src\main\resources\META-INF\persistence.xml在这里，需要配置数据源，数据源的位置是sqlite的db文件的位置D:\Database\sqlite\test1.db数据源的用户名和密码是空字符串就可以了

/module1/src/main/java/META-INF/persistence.xml

            yuki.jpa.hibernate.app1.module1.bean.Person                                                                                                        

 

3.SQLite的hibernate方言复制代码：https://code.google.com/p/hibernate-sqlite/source/browse/trunk/source/src/main/java/com/applerao/hibernatesqlite/dialect/SQLiteDialect.java?r=24修改报错内容：http://www.myexception.cn/open-source/1120141.html

hibernate4 替代hibernate3 的Hibernate.INTEGER的用法hibernate3 中没有了Hibernate.INTEGER的用法在Hibernate4中使用的方法是 StandardBasicTypes.INTEGER用法例如：sqlQuery.addScalar("id",StandardBasicTypes.INTEGER);

/module1/src/main/java/com/applerao/hibernatesqlite/dialect/SQLiteDialect.java

package com.applerao.hibernatesqlite.dialect;/** * https://code.google.com/p/hibernate-sqlite/source/browse/trunk/source/src/main/java/com/applerao/hibernatesqlite/dialect/SQLiteDialect.java?r=24 * http://www.myexception.cn/open-source/1120141.html *  *//* * The author disclaims copyright to this source code. In place of * a legal notice, here is a blessing: *  * May you do good and not evil. * May you find forgiveness for yourself and forgive others. * May you share freely, never taking more than you give. * */import java.sql.Types;import org.hibernate.dialect.Dialect;import org.hibernate.dialect.function.SQLFunctionTemplate;import org.hibernate.dialect.function.StandardSQLFunction;import org.hibernate.dialect.function.VarArgsSQLFunction;import org.hibernate.type.StandardBasicTypes;public class SQLiteDialect extends Dialect {    public SQLiteDialect() {        super();        registerColumnType(Types.BIT, "integer");        registerColumnType(Types.TINYINT, "tinyint");        registerColumnType(Types.SMALLINT, "smallint");        registerColumnType(Types.INTEGER, "integer");        registerColumnType(Types.BIGINT, "bigint");        registerColumnType(Types.FLOAT, "float");        registerColumnType(Types.REAL, "real");        registerColumnType(Types.DOUBLE, "double");        registerColumnType(Types.NUMERIC, "numeric");        registerColumnType(Types.DECIMAL, "decimal");        registerColumnType(Types.CHAR, "char");        registerColumnType(Types.VARCHAR, "varchar");        registerColumnType(Types.LONGVARCHAR, "longvarchar");        registerColumnType(Types.DATE, "date");        registerColumnType(Types.TIME, "time");        registerColumnType(Types.TIMESTAMP, "timestamp");        registerColumnType(Types.BINARY, "blob");        registerColumnType(Types.VARBINARY, "blob");        registerColumnType(Types.LONGVARBINARY, "blob");        // registerColumnType(Types.NULL, "null");        registerColumnType(Types.BLOB, "blob");        registerColumnType(Types.CLOB, "clob");        registerColumnType(Types.BOOLEAN, "integer");        registerFunction("concat", new VarArgsSQLFunction(StandardBasicTypes.STRING, "", "||", ""));        registerFunction("mod", new SQLFunctionTemplate(StandardBasicTypes.INTEGER, "?1 % ?2"));        registerFunction("substr", new StandardSQLFunction("substr", StandardBasicTypes.STRING));        registerFunction("substring", new StandardSQLFunction("substr", StandardBasicTypes.STRING));    }    public boolean supportsIdentityColumns() {        return true;    }    /*     * public boolean supportsInsertSelectIdentity() { return true; // As     * specify in NHibernate dialect }     */    public boolean hasDataTypeInIdentityColumn() {        return false; // As specify in NHibernate dialect    }    /*     * public String appendIdentitySelectToInsert(String insertString) { return     * new StringBuffer(insertString.length()+30). // As specify in NHibernate     * dialect append(insertString).     * append("; ").append(getIdentitySelectString()). toString(); }     */    public String getIdentityColumnString() {        // return "integer primary key autoincrement";        return "integer";    }    public String getIdentitySelectString() {        return "select last_insert_rowid()";    }    public boolean supportsLimit() {        return true;    }    public String getLimitString(String query, boolean hasOffset) {        return new StringBuffer(query.length() + 20).append(query).append(hasOffset ? " limit ? offset ?" : " limit ?").toString();    }    public boolean supportsTemporaryTables() {        return true;    }    public String getCreateTemporaryTableString() {        return "create temporary table if not exists";    }    public boolean dropTemporaryTableAfterUse() {        return false;    }    public boolean supportsCurrentTimestampSelection() {        return true;    }    public boolean isCurrentTimestampSelectStringCallable() {        return false;    }    public String getCurrentTimestampSelectString() {        return "select current_timestamp";    }    public boolean supportsUnionAll() {        return true;    }    public boolean hasAlterTable() {        return false; // As specify in NHibernate dialect    }    public boolean dropConstraints() {        return false;    }    public String getAddColumnString() {        return "add column";    }    public String getForUpdateString() {        return "";    }    public boolean supportsOuterJoinForUpdate() {        return false;    }    public String getDropForeignKeyString() {        throw new UnsupportedOperationException("No drop foreign key syntax supported by SQLiteDialect");    }    public String getAddForeignKeyConstraintString(String constraintName, String[] foreignKey, String referencedTable, String[] primaryKey, boolean referencesPrimaryKey) {        throw new UnsupportedOperationException("No add foreign key syntax supported by SQLiteDialect");    }    public String getAddPrimaryKeyConstraintString(String constraintName) {        throw new UnsupportedOperationException("No add primary key syntax supported by SQLiteDialect");    }    public boolean supportsIfExistsBeforeTableName() {        return true;    }    public boolean supportsCascadeDelete() {        return false;    }}

 

4.设置设置程序关联：控制面板\所有控制面板项\默认程序\设置默认程序\设置程序关联设置XML格式化代码时不换行：MyEclipse>FilesAndEditors>XML>XML Source>Linewidth=很大的值设置JAVA格式化代码时不换行：Java>CodeStyle>Formatter>New…>LineWrapping>SettingsForAnnotation>选择上面的选项>LineWrapPolicy=DoNotWrap

 

5.异常处理

没有持久化的提供者报错：javax.persistence.PersistenceException: No Persistence provider for EntityManager named module1解决：http://stackoverflow.com/questions/21975553/javax-persistence-persistenceexception-no-persistence-provider-for-entitymanage

If you're doing this from a JUnit test, and using maven,the persistence.xml should be located in src/test/resources/META-INF/persistence.xmlwhich will be put into the correct location at test execution time.The file in src/main/resources/META-INF/ is not used as it is not in the test-jar's path.

执行maven clean之后

执行maven clean后会删除运行处的persistence.xml，可以删除再添加persistence.xml，来使persistence.xml在运行的文件夹下

 

四、JPA实体1.自动建表http://www.cnblogs.com/talo/articles/1662244.html

validate　　　　加载hibernate时，验证创建数据库表结构create　　　　　每次加载hibernate，重新创建数据库表结构，这就是导致数据库表数据丢失的原因。create-drop　　加载hibernate时创建，退出是删除表结构update　　　　 加载hibernate自动更新数据库结构

需要建表的类Person.java添加注解Entity，被 @Entity注解的类就是JPA的实体类，主键注解 @Id， @GeneratedValuehttp://andyj.iteye.com/blog/287827

IDENTITY：　　　表自增键字段，Oracle不支持这种方式AUTO： 　　　 JPA自动选择合适的策略，是默认选项SEQUENCE：　　通过序列产生主键，通过 @SequenceGenerator注解指定序列名，MySql不支持这种方式TABLE：　　　　 通过表产生主键，框架借由表模拟序列产生主键，使用该策略可以使应用更易于数据库移植

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person1.txt

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;@Entitypublic class Person {    private Integer id;    private String name;    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    // @GeneratedValue (strategy=GenerationType.AUTO)    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

测试类PersonTest.java

获取实体管理器工厂，获取实体管理器，开启事务，保存，提交事务，关闭实体管理器，关闭实体管理器工厂

/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest1

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {    @Test    public void save() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager manager = factory.createEntityManager();        manager.getTransaction().begin();        manager.persist(new Person("person1"));        manager.getTransaction().commit();        manager.close();        factory.close();    }    }

刷新SQliteExpert中的数据库，看到新建的表和保存的数据

 

2.实体类的注解

自定义表名　　　　@Table(name="xxxPerson")自定义列的属性　　@Column(length=10, nullable=false, name="personName")

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person2.txt

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Table;@Entity@Table(name="xxxPerson")public class Person {    private Integer id;    private String name;    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(length=10, nullable=false, name="personName")    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

删除Person表，再次测试

日期类型　　@Temporal(TemporalType.DATE)枚举类型　　@Enumerated(EnumType.STRING)

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person3.txt

package yuki.jpa.hibernate.app1.module1.bean;import java.util.Date;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.EnumType;import javax.persistence.Enumerated;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Temporal;import javax.persistence.TemporalType;@Entitypublic class Person {    private Integer id;    private String name;    private Date birthday; //1987-12-10    private Gender gender = Gender.MAN; //默认值为MAN    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(length=10, nullable=false)    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }        @Temporal(TemporalType.DATE)    public Date getBirthday() {        return birthday;    }    public void setBirthday(Date birthday) {        this.birthday = birthday;    }        @Enumerated(EnumType.STRING)    @Column(length=5, nullable=false/*保存枚举值要设置数据库的字段不能为空*/)     public Gender getGender() {        return gender;    }    public void setGender(Gender gender) {        this.gender = gender;    }}

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/Gender.java

package yuki.jpa.hibernate.app1.module1.bean;public enum Gender {    MAN,WOMEN}

删除Person表，再次测试

文件或大文本数据　　@Lob

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person4.txt

package yuki.jpa.hibernate.app1.module1.bean;import java.util.Date;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.EnumType;import javax.persistence.Enumerated;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Lob;import javax.persistence.Temporal;import javax.persistence.TemporalType;@Entitypublic class Person {    private Integer id;    private String name;    private Date birthday; //1987-12-10    private Gender gender = Gender.MAN; //默认值为MAN    private String info; //存放大文本数据    private byte[] file; //存放文件    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(length=10, nullable=false)    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }        @Temporal(TemporalType.DATE)    public Date getBirthday() {        return birthday;    }    public void setBirthday(Date birthday) {        this.birthday = birthday;    }        @Enumerated(EnumType.STRING)    @Column(length=5, nullable=false/*保存枚举值要设置数据库的字段不能为空*/)     public Gender getGender() {        return gender;    }    public void setGender(Gender gender) {        this.gender = gender;    }        @Lob    public String getInfo() {        return info;    }    public void setInfo(String info) {        this.info = info;    }        @Lob    public byte[] getFile() {        return file;    }    public void setFile(byte[] file) {        this.file = file;    }}

延迟加载 　　　　　　@Basic(fetch=FetchType.LAZY)不持久化的字段　　　@Transient

/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person5.txt

package yuki.jpa.hibernate.app1.module1.bean;import java.util.Date;import javax.persistence.Basic;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.EnumType;import javax.persistence.Enumerated;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Lob;import javax.persistence.Temporal;import javax.persistence.TemporalType;import javax.persistence.Transient;@Entitypublic class Person {    private Integer id;    private String name;    private Date birthday; //1987-12-10    private Gender gender = Gender.MAN; //默认值为MAN    private String info; //存放大文本    private byte[] file; //存放大文件，延迟加载    private String imagePath; //不持久化的字段    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(length=10, nullable=false)    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }        @Temporal(TemporalType.DATE)    public Date getBirthday() {        return birthday;    }    public void setBirthday(Date birthday) {        this.birthday = birthday;    }        @Enumerated(EnumType.STRING)    @Column(length=5, nullable=false/*保存枚举值要设置数据库的字段不能为空*/)     public Gender getGender() {        return gender;    }    public void setGender(Gender gender) {        this.gender = gender;    }        @Lob    public String getInfo() {        return info;    }    public void setInfo(String info) {        this.info = info;    }        @Lob    @Basic(fetch=FetchType.LAZY) //访问这个属性的时候才会加载进内存    public byte[] getFile() {        return file;    }    public void setFile(byte[] file) {        this.file = file;    }        @Transient    public String getImagePath() {        return imagePath;    }    public void setImagePath(String imagePath) {        this.imagePath = imagePath;    }}

下面使用的实体类/module1/src/main/java/yuki/jpa/hibernate/app1/module1/bean/person/Person7

package yuki.jpa.hibernate.app1.module1.bean;import java.util.Date;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.EnumType;import javax.persistence.Enumerated;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Temporal;import javax.persistence.TemporalType;import javax.persistence.Transient;@Entitypublic class Person {    private Integer id;    private String name;    private Date birthday; //1987-12-10    private Gender gender = Gender.MAN; //默认值为MAN    private String imagePath; //不持久化的字段    public Person() {}    public Person(String name) {        this.name = name;    }    @Id    @GeneratedValue    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(length=10, nullable=false)    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }        @Temporal(TemporalType.DATE)    public Date getBirthday() {        return birthday;    }    public void setBirthday(Date birthday) {        this.birthday = birthday;    }        @Enumerated(EnumType.STRING)    @Column(length=5, nullable=false/*保存枚举值要设置数据库的字段不能为空*/)     public Gender getGender() {        return gender;    }    public void setGender(Gender gender) {        this.gender = gender;    }        @Transient    public String getImagePath() {        return imagePath;    }    public void setImagePath(String imagePath) {        this.imagePath = imagePath;    }        @Override    public String toString() {        return "Person [id=" + id + ", name=" + name + ", birthday=" + birthday + ", gender=" + gender + ", imagePath=" + imagePath + "]";    }}

 

3.对实体类的查找/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest2

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {        @Test    public void getPerson() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager em = factory.createEntityManager();//        em.getTransaction().begin();        Person person = em.find(Person.class, 1);        System.out.println(person);//        em.getTransaction().commit();        em.close();        factory.close();    }}

Hibernate: select person0_.id as id1_0_0_, person0_.birthday as birthday2_0_0_, person0_.gender as gender3_0_0_, person0_.name as name4_0_0_ from Person person0_ where person0_.id=?Person [id=1, name=person1, birthday=null, gender=MAN, imagePath=null]

 

.find和.getReference/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest4

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {    @Test    public void getPerson() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager em = factory.createEntityManager();//        Person person = em.find(Person.class, 1); //相当于hibernate的get        Person person = em.getReference(Person.class, 1); //相当于hibernate的load//        load之后返回的是代理对象，使用的是cglib动态创建字节码的技术//        如果不访问数据，是不回发生数据的加载行为的//        org.hibernate.LazyInitializationException: could not initialize proxy - no session        em.close();        System.out.println(person.getName());        factory.close();    }    }

Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 2.712 sec <<< FAILURE!getPerson(yuki.jpa.hibernate.app1.module1.bean.PersonTest) Time elapsed: 2.613 sec <<< ERROR!org.hibernate.LazyInitializationException: could not initialize proxy - no Session

 

异常在访问引用对象的属性时就会发生，即使实体管理器没有关闭/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest5

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {    @Test    public void getPerson() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager em = factory.createEntityManager();        /*Person person = em.find(Person.class, 2);//null        Person person = em.getReference(Person.class, 2);//javax.persistence.EntityNotFoundException        System.out.println(person);*/        Person person = em.getReference(Person.class, 2);        System.out.println(person.getName()); //这里出现了异常        em.close();        factory.close();    }    }

at yuki.jpa.hibernate.app1.module1.bean.Person_$$_jvst586_0.getName(Person_$$_jvst586_0.java)at yuki.jpa.hibernate.app1.module1.bean.PersonTest.getPerson(PersonTest.java:19)

 

4.JPA实体的四种状态

new(新建)、managed(托管)、游离(脱管)、删除当一个对象与事务关联且进入托管状态时对属性进行更新，对应的数据就会同步到数据库对象的属性发生更改后，会放进jdbc的批提交里去

/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest6

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {    @Test    public void updatePerson() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager em = factory.createEntityManager();        em.getTransaction().begin();                Person person = em.find(Person.class, 2);        person.setName("老张");                em.getTransaction().commit();        em.close();        factory.close();                /**         * JPA对象的四种状态         * new(新建)         * managed(托管)         * 游离(脱管)         * 删除         *          * 当一个对象与事务关联且进入托管状态时，         * 对属性进行更新，对应的数据就会同步到数据库         * 对象的属性发生更改后，会放进jdbc的批提交里去         *          */    }    }

把实体管理器中的所有实体变成游离对象　　　.clear()把游离对象同步回数据库　　　　　　　　 .merge(person)

/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest7

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;public class PersonTest {    @Test    public void updatePerson() {        EntityManagerFactory factory = Persistence.createEntityManagerFactory("module1");        EntityManager em = factory.createEntityManager();        em.getTransaction().begin();                Person person = em.find(Person.class, 2);        em.clear(); //把实体管理器中的所有实体变成游离对象        person.setName("老黎");        em.merge(person); //把游离对象同步回数据库                em.getTransaction().commit();        em.close();        factory.close();    }    }

删除实体/module1/src/test/java/yuki/jpa/hibernate/app1/module1/bean/personTest/PersonTest8

package yuki.jpa.hibernate.app1.module1.bean;import javax.persistence.EntityManager;import ja

转载于:https://www.cnblogs.com/summary-2017/p/7389329.html