第16章 Spring Boot + Kotlin： 下一代 Java 服务端开发
16.1 Kotlin 简介
16.2 Kotlin 与 Java 的无缝集成
16.3 Spring Boot 集成 Kotlin 开发实例详解
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Nextj2eeApplication.kt
package com.kotlin.springboot.nextj2ee
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
@SpringBootApplication
class Nextj2eeApplication
fun main(args: Array<String>) {
runApplication<Nextj2eeApplication>(*args)
}
build.gradle
buildscript {
ext {
kotlinVersion = '1.1.60'
springBootVersion = '2.0.0.M6'
}
repositories {
mavenCentral()
maven { url "https://repo.spring.io/snapshot" }
maven { url "https://repo.spring.io/milestone" }
}
dependencies {
classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}")
classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlinVersion}")
}
}
apply plugin: 'kotlin'
apply plugin: 'kotlin-spring'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'
group = 'com.kotlin.springboot'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8
compileKotlin {
kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
kotlinOptions.jvmTarget = "1.8"
}
repositories {
mavenCentral()
maven { url "https://repo.spring.io/snapshot" }
maven { url "https://repo.spring.io/milestone" }
}
dependencies {
compile('org.springframework.boot:spring-boot-starter-data-jpa')
compile('org.springframework.boot:spring-boot-starter-freemarker')
compile('org.springframework.boot:spring-boot-starter-web')
compile("org.jetbrains.kotlin:kotlin-stdlib-jre8")
compile("org.jetbrains.kotlin:kotlin-reflect")
runtime('mysql:mysql-connector-java')
testCompile('org.springframework.boot:spring-boot-starter-test')
}
Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaConfiguration': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Hikari.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.zaxxer.hikari.HikariDataSource]: Factory method 'dataSource' threw exception; nested exception is org.springframework.boot.autoconfigure.jdbc.DataSourceProperties$DataSourceBeanCreationException: Cannot determine embedded database driver class for database type NONE. If you want an embedded database please put a supported one on the classpath. If you have database settings to be loaded from a particular profile you may need to active it (no profiles are currently active).
2017-11-22 10:42:57.949 INFO 13877 --- [ main] o.apache.catalina.core.StandardService : Stopping service [Tomcat]
2017-11-22 10:42:58.000 INFO 13877 --- [ main] utoConfigurationReportLoggingInitializer :
Error starting ApplicationContext. To display the auto-configuration report re-run your application with 'debug' enabled.
2017-11-22 10:42:58.004 ERROR 13877 --- [ main] o.s.b.d.LoggingFailureAnalysisReporter :
***************************
APPLICATION FAILED TO START
***************************
Description:
Cannot determine embedded database driver class for database type NONE
Action:
If you want an embedded database please put a supported one on the classpath. If you have database settings to be loaded from a particular profile you may need to active it (no profiles are currently active).
Process finished with exit code 1
BeanCreationException: Error creating bean with name 'dataSource' defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Hikari.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.zaxxer.hikari.HikariDataSource]: Factory method 'dataSource' threw exception; nested exception is org.springframework.boot.autoconfigure.jdbc.DataSourceProperties$DataSourceBeanCreationException: Cannot determine embedded database driver class for database type NONE. If you want an embedded database please put a supported one on the classpath. If you have database settings to be loaded from a particular profile you may need to active it (no profiles are currently active).
Database dataSource Config
2017-11-22 11:26:24.961 INFO 14423 --- [ main] com.zaxxer.hikari.HikariDataSource : testdb - Starting...
2017-11-22 11:26:25.306 INFO 14423 --- [ main] com.zaxxer.hikari.HikariDataSource : testdb - Start completed.
2017-11-22 11:26:25.373 INFO 14423 --- [ main] j.LocalContainerEntityManagerFactoryBean : Building JPA container EntityManagerFactory for persistence unit 'default'
2017-11-22 11:26:25.385 INFO 14423 --- [ main] o.hibernate.jpa.internal.util.LogHelper : HHH000204: Processing PersistenceUnitInfo [
name: default
...]
2017-11-22 11:26:25.481 INFO 14423 --- [ main] org.hibernate.Version : HHH000412: Hibernate Core {5.2.12.Final}
2017-11-22 11:26:25.485 INFO 14423 --- [ main] org.hibernate.cfg.Environment : HHH000206: hibernate.properties not found
2017-11-22 11:26:25.544 INFO 14423 --- [ main] o.hibernate.annotations.common.Version : HCANN000001: Hibernate Commons Annotations {5.0.1.Final}
2017-11-22 11:26:25.738 INFO 14423 --- [ main] org.hibernate.dialect.Dialect : HHH000400: Using dialect: org.hibernate.dialect.MySQL5Dialect
2017-11-22 11:26:26.020 INFO 14423 --- [ main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit 'default'
hibernate.properties
#
# Hibernate, Relational Persistence for Idiomatic Java
# org.hibernate.cfg.AvailableSettings
#
# License: GNU Lesser General Public License (LGPL), version 2.1 or later.
# See the lgpl.txt file in the root directory or <http://www.gnu.org/licenses/lgpl-2.1.html>.
#
######################
### 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/
2017-11-22 11:55:17.205 INFO 14721 --- [ main] org.hibernate.Version : HHH000412: Hibernate Core {5.2.12.Final}
2017-11-22 11:55:17.208 INFO 14721 --- [ main] org.hibernate.cfg.Environment : HHH000205: Loaded properties from resource hibernate.properties: {hibernate.connection.driver_class=com.mysql.jdbc.Driver, hibernate.format_sql=true, hibernate.dialect=org.hibernate.dialect.MySQLDialect, hibernate.bytecode.use_reflection_optimizer=false, hibernate.max_fetch_depth=1, hibernate.connection.pool_size=1}
2017-11-22 11:55:17.261 INFO 14721 --- [ main] o.hibernate.annotations.common.Version : HCANN000001: Hibernate Commons Annotations {5.0.1.Final}
2017-11-22 11:55:17.449 INFO 14721 --- [ main] org.hibernate.dialect.Dialect : HHH000400: Using dialect: org.hibernate.dialect.MySQL5Dialect
2017-11-22 11:55:17.665 INFO 14721 --- [ main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit 'default'
Article.kt
package com.kotlin.springboot.nextj2ee.entity
import java.util.*
import javax.persistence.*
@Entity
class Article {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
var id: Long = -1
lateinit var author: String
lateinit var title: String
@Column(name = "url", unique = true, length = 512)
lateinit var url: String
@Lob
lateinit var content: String
lateinit var gmtCreate: Date
lateinit var gmtModify: Date
}
对应的Hibernate 启动日志
Hibernate:
drop table if exists article
Hibernate:
create table article (
id bigint not null auto_increment,
author varchar(255),
content longtext,
gmt_create datetime,
gmt_modify datetime,
title varchar(255),
url varchar(250),
primary key (id)
) engine=MyISAM
Hibernate:
alter table article
add constraint UK_la4e9l2g2otn85r9ih4btnmi unique (url)
url varchar(250)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Specified key was too long; max key length is 1000 bytes
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:na]
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:488) ~[na:na]
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.Util.getInstance(Util.java:408) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:943) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3973) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3909) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2527) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2680) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2483) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2441) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.StatementImpl.executeInternal(StatementImpl.java:845) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:745) ~[mysql-connector-java-5.1.44.jar:5.1.44]
at com.zaxxer.hikari.pool.ProxyStatement.execute(ProxyStatement.java:95) ~[HikariCP-2.7.2.jar:na]
at com.zaxxer.hikari.pool.HikariProxyStatement.execute(HikariProxyStatement.java) ~[HikariCP-2.7.2.jar:na]
at org.hibernate.tool.schema.internal.exec.GenerationTargetToDatabase.accept(GenerationTargetToDatabase.java:54) ~[hibernate-core-5.2.12.Final.jar:5.2.12.Final]
... 33 common frames omitted
由 Entity 类生成的表结构：
image.png
唯一索引信息：
image.png
MySQL 的 utf8mb4 （most bytes 4）编码
一、简介
MySQL在5.5.3之后增加了这个utf8mb4的编码，mb4就是most bytes 4的意思，专门用来兼容四字节的unicode。好在utf8mb4是utf8的超集，除了将编码改为utf8mb4外不需要做其他转换。当然，为了节省空间，一般情况下使用utf8也就够了。
二、内容描述
那上面说了既然utf8能够存下大部分中文汉字,那为什么还要使用utf8mb4呢? 原来mysql支持的 utf8 编码最大字符长度为 3 字节，如果遇到 4 字节的宽字符就会插入异常了。三个字节的 UTF-8 最大能编码的 Unicode 字符是 0xffff，也就是 Unicode 中的基本多文种平面（BMP）。也就是说，任何不在基本多文本平面的 Unicode字符，都无法使用 Mysql 的 utf8 字符集存储。包括 Emoji 表情（Emoji 是一种特殊的 Unicode 编码，常见于 ios 和 android 手机上），和很多不常用的汉字，以及任何新增的 Unicode 字符等等。
三、问题根源
最初的 UTF-8 格式使用一至六个字节，最大能编码 31 位字符。最新的 UTF-8 规范只使用一到四个字节，最大能编码21位，正好能够表示所有的 17个 Unicode 平面。
utf8 是 Mysql 中的一种字符集，只支持最长三个字节的 UTF-8字符，也就是 Unicode 中的基本多文本平面。
Mysql 中的 utf8 为什么只支持持最长三个字节的 UTF-8字符呢？我想了一下，可能是因为 Mysql 刚开始开发那会，Unicode 还没有辅助平面这一说呢。那时候，Unicode 委员会还做着 “65535 个字符足够全世界用了”的美梦。Mysql 中的字符串长度算的是字符数而非字节数，对于 CHAR 数据类型来说，需要为字符串保留足够的长。当使用 utf8 字符集时，需要保留的长度就是 utf8 最长字符长度乘以字符串长度，所以这里理所当然的限制了 utf8 最大长度为 3，比如 CHAR(100) Mysql 会保留 300字节长度。至于后续的版本为什么不对 4 字节长度的 UTF-8 字符提供支持，我想一个是为了向后兼容性的考虑，还有就是基本多文种平面之外的字符确实很少用到。
要在 Mysql 中保存 4 字节长度的 UTF-8 字符，需要使用 utf8mb4 字符集，但只有 5.5.3 版本以后的才支持（查看版本： select version();）。我觉得，为了获取更好的兼容性，应该总是使用 utf8mb4 而非 utf8. 对于 CHAR 类型数据，utf8mb4 会多消耗一些空间，根据 Mysql 官方建议，使用 VARCHAR 替代 CHAR。
16.4 本章小结
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