@Column注解详解:映射数据库字段

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"本文详细介绍了Java Persistence API (JPA) 中的`@Column`注解,以及如何使用它来映射实体类的属性到数据库表的字段。内容包括`@Column`注解的各种属性及其含义,同时也探讨了在实体类中应用`@Column`的注意事项。" 在Java开发中,特别是在使用JPA进行对象关系映射(ORM)时,`@Column`注解是至关重要的。这个注解用于指定实体类中的属性如何映射到数据库表的列。`@Column`注解包含多个属性,允许开发者精确控制字段的行为。 1. `name`: 这个属性定义了映射到数据库表的列名,默认为空,即使用属性或方法名称作为列名。如果需要自定义列名,可以设置此属性。 2. `unique`: 如果设置为`true`,则表示该字段的值必须是唯一的,数据库会添加一个唯一性约束。默认为`false`。 3. `nullable`: 指定该字段是否允许存储`null`值。默认值为`true`,允许`null`。如果设置为`false`,数据库将创建一个非空约束。 4. `insertable`: 此属性控制在执行`INSERT`操作时,是否将该字段的值写入数据库。默认为`true`,表示会插入。如果设置为`false`,则在插入新记录时,该字段不会被包含在SQL语句中。 5. `updatable`: 类似于`insertable`,`updatable`属性决定在`UPDATE`操作时,是否更新该字段的值。默认为`true`,表示可更新。如果设置为`false`,则在更新记录时,该字段不会被包含在SQL语句中。 6. `columnDefinition`: 这个属性允许直接指定列的SQL定义,例如数据类型、长度等。这在需要特定数据库特性时非常有用,如自增序列或特定的数据类型。 7. `table`: 用于指定该列所属的表,当属性属于子类继承的父类时,可能需要明确指定表名。 8. `length`, `precision`, `scale`: 分别用于定义字符类型的长度、数值类型的精度和小数位数。 在实际应用中,`@Column`注解可以用于字段或其对应的getter方法,JPA规范并未强制要求使用哪种方式,开发者可以根据自己的习惯选择。值得注意的是,对于主键和外键等只读属性,通常会设置`insertable`和`updatable`为`false`,以防止它们在数据库操作中被意外修改。 `@Column`注解提供了丰富的选项来定制实体类属性与数据库表字段之间的映射关系,帮助开发者更精确地控制数据存储行为。理解和熟练使用`@Column`注解,对于优化JPA应用的性能和数据完整性至关重要。
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hibernate.properties # # Hibernate, Relational Persistence for Idiomatic Java # # License: GNU Lesser General Public License (LGPL), version 2.1 or later. # See the lgpl.txt file in the root directory or . # ###################### ### Query Language ### ###################### ## define query language constants / function names hibernate.query.substitutions yes 'Y', no 'N' ## select the classic query parser #hibernate.query.factory_class org.hibernate.hql.internal.classic.ClassicQueryTranslatorFactory ################# ### Platforms ### ################# ## JNDI Datasource #hibernate.connection.datasource jdbc/test #hibernate.connection.username db2 #hibernate.connection.password db2 ## HypersonicSQL hibernate.dialect org.hibernate.dialect.HSQLDialect hibernate.connection.driver_class org.hsqldb.jdbcDriver hibernate.connection.username sa hibernate.connection.password hibernate.connection.url jdbc:hsqldb:./build/db/hsqldb/hibernate #hibernate.connection.url jdbc:hsqldb:hsql://localhost #hibernate.connection.url jdbc:hsqldb:test ## H2 (www.h2database.com) #hibernate.dialect org.hibernate.dialect.H2Dialect #hibernate.connection.driver_class org.h2.Driver #hibernate.connection.username sa #hibernate.connection.password #hibernate.connection.url jdbc:h2:mem:./build/db/h2/hibernate #hibernate.connection.url jdbc:h2:testdb/h2test #hibernate.connection.url jdbc:h2:mem:imdb1 #hibernate.connection.url jdbc:h2:tcp://dbserv:8084/sample; #hibernate.connection.url jdbc:h2:ssl://secureserv:8085/sample; #hibernate.connection.url jdbc:h2:ssl://secureserv/testdb;cipher=AES ## MySQL #hibernate.dialect org.hibernate.dialect.MySQLDialect #hibernate.dialect org.hibernate.dialect.MySQLInnoDBDialect #hibernate.dialect org.hibernate.dialect.MySQLMyISAMDialect #hibernate.connection.driver_class com.mysql.jdbc.Driver #hibernate.connection.url jdbc:mysql:///test #hibernate.connection.username gavin #hibernate.connection.password ## Oracle #hibernate.dialect org.hibernate.dialect.Oracle8iDialect #hibernate.dialect org.hibernate.dialect.Oracle9iDialect #hibernate.dialect org.hibernate.dialect.Oracle10gDialect #hibernate.connection.driver_class oracle.jdbc.driver.OracleDriver #hibernate.connection.username ora #hibernate.connection.password ora #hibernate.connection.url jdbc:oracle:thin:@localhost:1521:orcl #hibernate.connection.url jdbc:oracle:thin:@localhost:1522:XE ## PostgreSQL #hibernate.dialect org.hibernate.dialect.PostgreSQLDialect #hibernate.connection.driver_class org.postgresql.Driver #hibernate.connection.url jdbc:postgresql:template1 #hibernate.connection.username pg #hibernate.connection.password ## DB2 #hibernate.dialect org.hibernate.dialect.DB2Dialect #hibernate.connection.driver_class com.ibm.db2.jcc.DB2Driver #hibernate.connection.driver_class COM.ibm.db2.jdbc.app.DB2Driver #hibernate.connection.url jdbc:db2://localhost:50000/somename #hibernate.connection.url jdbc:db2:somename #hibernate.connection.username db2 #hibernate.connection.password db2 ## TimesTen #hibernate.dialect org.hibernate.dialect.TimesTenDialect #hibernate.connection.driver_class com.timesten.jdbc.TimesTenDriver #hibernate.connection.url jdbc:timesten:direct:test #hibernate.connection.username #hibernate.connection.password ## DB2/400 #hibernate.dialect org.hibernate.dialect.DB2400Dialect #hibernate.connection.username user #hibernate.connection.password password ## Native driver #hibernate.connection.driver_class COM.ibm.db2.jdbc.app.DB2Driver #hibernate.connection.url jdbc:db2://systemname ## Toolbox driver #hibernate.connection.driver_class com.ibm.as400.access.AS400JDBCDriver #hibernate.connection.url jdbc:as400://systemname ## Derby (not supported!) #hibernate.dialect org.hibernate.dialect.DerbyDialect #hibernate.connection.driver_class org.apache.derby.jdbc.EmbeddedDriver #hibernate.connection.username #hibernate.connection.password #hibernate.connection.url jdbc:derby:build/db/derby/hibernate;create=true ## Sybase #hibernate.dialect org.hibernate.dialect.SybaseDialect #hibernate.connection.driver_class com.sybase.jdbc2.jdbc.SybDriver #hibernate.connection.username sa #hibernate.connection.password sasasa #hibernate.connection.url jdbc:sybase:Tds:co3061835-a:5000/tempdb ## Mckoi SQL #hibernate.dialect org.hibernate.dialect.MckoiDialect #hibernate.connection.driver_class com.mckoi.JDBCDriver #hibernate.connection.url jdbc:mckoi:/// #hibernate.connection.url jdbc:mckoi:local://C:/mckoi1.0.3/db.conf #hibernate.connection.username admin #hibernate.connection.password nimda ## SAP DB #hibernate.dialect org.hibernate.dialect.SAPDBDialect #hibernate.connection.driver_class com.sap.dbtech.jdbc.DriverSapDB #hibernate.connection.url jdbc:sapdb://localhost/TST #hibernate.connection.username TEST #hibernate.connection.password TEST #hibernate.query.substitutions yes 'Y', no 'N' ## MS SQL Server #hibernate.dialect org.hibernate.dialect.SQLServerDialect #hibernate.connection.username sa #hibernate.connection.password sa ## JSQL Driver #hibernate.connection.driver_class com.jnetdirect.jsql.JSQLDriver #hibernate.connection.url jdbc:JSQLConnect://1E1/test ## JTURBO Driver #hibernate.connection.driver_class com.newatlanta.jturbo.driver.Driver #hibernate.connection.url jdbc:JTurbo://1E1:1433/test ## WebLogic Driver #hibernate.connection.driver_class weblogic.jdbc.mssqlserver4.Driver #hibernate.connection.url jdbc:weblogic:mssqlserver4:1E1:1433 ## Microsoft Driver (not recommended!) #hibernate.connection.driver_class com.microsoft.jdbc.sqlserver.SQLServerDriver #hibernate.connection.url jdbc:microsoft:sqlserver://1E1;DatabaseName=test;SelectMethod=cursor ## The New Microsoft Driver #hibernate.connection.driver_class com.microsoft.sqlserver.jdbc.SQLServerDriver #hibernate.connection.url jdbc:sqlserver://localhost ## jTDS (since version 0.9) #hibernate.connection.driver_class net.sourceforge.jtds.jdbc.Driver #hibernate.connection.url jdbc:jtds:sqlserver://1E1/test ## Interbase #hibernate.dialect org.hibernate.dialect.InterbaseDialect #hibernate.connection.username sysdba #hibernate.connection.password masterkey ## DO NOT specify hibernate.connection.sqlDialect ## InterClient #hibernate.connection.driver_class interbase.interclient.Driver #hibernate.connection.url jdbc:interbase://localhost:3060/C:/firebird/test.gdb ## Pure Java #hibernate.connection.driver_class org.firebirdsql.jdbc.FBDriver #hibernate.connection.url jdbc:firebirdsql:localhost/3050:/firebird/test.gdb ## Pointbase #hibernate.dialect org.hibernate.dialect.PointbaseDialect #hibernate.connection.driver_class com.pointbase.jdbc.jdbcUniversalDriver #hibernate.connection.url jdbc:pointbase:embedded:sample #hibernate.connection.username PBPUBLIC #hibernate.connection.password PBPUBLIC ## Ingres ## older versions (before Ingress 2006) #hibernate.dialect org.hibernate.dialect.IngresDialect #hibernate.connection.driver_class ca.edbc.jdbc.EdbcDriver #hibernate.connection.url jdbc:edbc://localhost:II7/database #hibernate.connection.username user #hibernate.connection.password password ## Ingres 2006 or later #hibernate.dialect org.hibernate.dialect.IngresDialect #hibernate.connection.driver_class com.ingres.jdbc.IngresDriver #hibernate.connection.url jdbc:ingres://localhost:II7/database;CURSOR=READONLY;auto=multi #hibernate.connection.username user #hibernate.connection.password password ## Mimer SQL #hibernate.dialect org.hibernate.dialect.MimerSQLDialect #hibernate.connection.driver_class com.mimer.jdbc.Driver #hibernate.connection.url jdbc:mimer:multi1 #hibernate.connection.username hibernate #hibernate.connection.password hibernate ## InterSystems Cache #hibernate.dialect org.hibernate.dialect.Cache71Dialect #hibernate.connection.driver_class com.intersys.jdbc.CacheDriver #hibernate.connection.username _SYSTEM #hibernate.connection.password SYS #hibernate.connection.url jdbc:Cache://127.0.0.1:1972/HIBERNATE ################################# ### Hibernate Connection Pool ### ################################# hibernate.connection.pool_size 1 ########################### ### C3P0 Connection Pool### ########################### #hibernate.c3p0.max_size 2 #hibernate.c3p0.min_size 2 #hibernate.c3p0.timeout 5000 #hibernate.c3p0.max_statements 100 #hibernate.c3p0.idle_test_period 3000 #hibernate.c3p0.acquire_increment 2 #hibernate.c3p0.validate false ############################## ### Proxool Connection Pool### ############################## ## Properties for external configuration of Proxool hibernate.proxool.pool_alias pool1 ## Only need one of the following #hibernate.proxool.existing_pool true #hibernate.proxool.xml proxool.xml #hibernate.proxool.properties proxool.properties ################################# ### Plugin ConnectionProvider ### ################################# ## use a custom ConnectionProvider (if not set, Hibernate will choose a built-in ConnectionProvider using hueristics) #hibernate.connection.provider_class org.hibernate.connection.DriverManagerConnectionProvider #hibernate.connection.provider_class org.hibernate.connection.DatasourceConnectionProvider #hibernate.connection.provider_class org.hibernate.connection.C3P0ConnectionProvider #hibernate.connection.provider_class org.hibernate.connection.ProxoolConnectionProvider ####################### ### Transaction API ### ####################### ## Enable automatic flush during the JTA beforeCompletion() callback ## (This setting is relevant with or without the Transaction API) #hibernate.transaction.flush_before_completion ## Enable automatic session close at the end of transaction ## (This setting is relevant with or without the Transaction API) #hibernate.transaction.auto_close_session ## the Transaction API abstracts application code from the underlying JTA or JDBC transactions #hibernate.transaction.factory_class org.hibernate.transaction.JTATransactionFactory #hibernate.transaction.factory_class org.hibernate.transaction.JDBCTransactionFactory ## to use JTATransactionFactory, Hibernate must be able to locate the UserTransaction in JNDI ## default is java:comp/UserTransaction ## you do NOT need this setting if you specify hibernate.transaction.manager_lookup_class #jta.UserTransaction jta/usertransaction #jta.UserTransaction javax.transaction.UserTransaction #jta.UserTransaction UserTransaction ## to use the second-level cache with JTA, Hibernate must be able to obtain the JTA TransactionManager #hibernate.transaction.manager_lookup_class org.hibernate.transaction.JBossTransactionManagerLookup #hibernate.transaction.manager_lookup_class org.hibernate.transaction.WeblogicTransactionManagerLookup #hibernate.transaction.manager_lookup_class org.hibernate.transaction.WebSphereTransactionManagerLookup #hibernate.transaction.manager_lookup_class org.hibernate.transaction.OrionTransactionManagerLookup #hibernate.transaction.manager_lookup_class org.hibernate.transaction.ResinTransactionManagerLookup ############################## ### Miscellaneous Settings ### ############################## ## print all generated SQL to the console #hibernate.show_sql true ## format SQL in log and console hibernate.format_sql true ## add comments to the generated SQL #hibernate.use_sql_comments true ## generate statistics #hibernate.generate_statistics true ## auto schema export #hibernate.hbm2ddl.auto create-drop #hibernate.hbm2ddl.auto create #hibernate.hbm2ddl.auto update #hibernate.hbm2ddl.auto validate ## specify a default schema and catalog for unqualified tablenames #hibernate.default_schema test #hibernate.default_catalog test ## enable ordering of SQL UPDATEs by primary key #hibernate.order_updates true ## set the maximum depth of the outer join fetch tree hibernate.max_fetch_depth 1 ## set the default batch size for batch fetching #hibernate.default_batch_fetch_size 8 ## rollback generated identifier values of deleted entities to default values #hibernate.use_identifier_rollback true ## enable bytecode reflection optimizer (disabled by default) #hibernate.bytecode.use_reflection_optimizer true ##################### ### JDBC Settings ### ##################### ## specify a JDBC isolation level #hibernate.connection.isolation 4 ## enable JDBC autocommit (not recommended!) #hibernate.connection.autocommit true ## set the JDBC fetch size #hibernate.jdbc.fetch_size 25 ## set the maximum JDBC 2 batch size (a nonzero value enables batching) #hibernate.jdbc.batch_size 5 #hibernate.jdbc.batch_size 0 ## enable batch updates even for versioned data hibernate.jdbc.batch_versioned_data true ## enable use of JDBC 2 scrollable ResultSets (specifying a Dialect will cause Hibernate to use a sensible default) #hibernate.jdbc.use_scrollable_resultset true ## use streams when writing binary types to / from JDBC hibernate.jdbc.use_streams_for_binary true ## use JDBC 3 PreparedStatement.getGeneratedKeys() to get the identifier of an inserted row #hibernate.jdbc.use_get_generated_keys false ## choose a custom JDBC batcher # hibernate.jdbc.factory_class ## enable JDBC result set column alias caching ## (minor performance enhancement for broken JDBC drivers) # hibernate.jdbc.wrap_result_sets ## choose a custom SQL exception converter #hibernate.jdbc.sql_exception_converter ########################## ### Second-level Cache ### ########################## ## optimize cache for minimal "puts" instead of minimal "gets" (good for clustered cache) #hibernate.cache.use_minimal_puts true ## set a prefix for cache region names hibernate.cache.region_prefix hibernate.test ## disable the second-level cache #hibernate.cache.use_second_level_cache false ## enable the query cache #hibernate.cache.use_query_cache true ## store the second-level cache entries in a more human-friendly format #hibernate.cache.use_structured_entries true ## choose a cache implementation #hibernate.cache.region.factory_class org.hibernate.cache.infinispan.InfinispanRegionFactory #hibernate.cache.region.factory_class org.hibernate.cache.infinispan.JndiInfinispanRegionFactory #hibernate.cache.region.factory_class org.hibernate.cache.internal.EhCacheRegionFactory #hibernate.cache.region.factory_class org.hibernate.cache.internal.SingletonEhCacheRegionFactory hibernate.cache.region.factory_class org.hibernate.cache.internal.NoCachingRegionFactory ## choose a custom query cache implementation #hibernate.cache.query_cache_factory ############ ### JNDI ### ############ ## specify a JNDI name for the SessionFactory #hibernate.session_factory_name hibernate/session_factory ## Hibernate uses JNDI to bind a name to a SessionFactory and to look up the JTA UserTransaction; ## if hibernate.jndi.* are not specified, Hibernate will use the default InitialContext() which ## is the best approach in an application server #file system #hibernate.jndi.class com.sun.jndi.fscontext.RefFSContextFactory #hibernate.jndi.url file:/ #WebSphere #hibernate.jndi.class com.ibm.websphere.naming.WsnInitialContextFactory #hibernate.jndi.url iiop://localhost:900/