A Clojure library designed to ... well, that part is up to you.
Java 提供了标准的 JDBC 接口访问数据库，Clojure 的数据库接口
clojure.java.jdbc是对 Java JDBC 的封装。我们只需要引用 clojure.java.jdbc 以及对应的数据库驱动，就可以在 Clojure 代码中访问数据库。
clojure.java.jdbc 是一个比较底层的接口。我们使用 DSL 的模式来编写数据库代码，类似 Java 的 Hibernate，选择 Korma 来编写访问数据库的代码。
由于 Clojure 是 Lisp 方言，它继承了 Lisp 强大的“代码即数据”的功能，在 Clojure 代码中，编写 SQL 语句对应的 DSL 十分自然，完全无需 Hibernate 复杂的映射配置。
我们先配置好 MySQL 数据库，然后创建一个表来测试 Clojure 代码.
创建 schema
CREATE SCHEMA `cljweb` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci ;
创建表
create table courses (
`id` varchar(32) not null primary key,
`name` varchar(50) not null,
`price` real not null,
`online` bool not null,
`days` bigint not null
);
新建一个 db.clj 文件，选择菜单“File”-“New”-“Other...”，选择“Clojure”-“Clojure Namespace”，填入名称
course.db
就可以创建一个 db.clj 文件。
在编写代码前，我们首先要在 project.clj 文件中添加依赖项。
[org.clojure/java.jdbc "0.3.6"]
[mysql/mysql-connector-java "5.1.25"]
[korma "0.3.0"]我们执行
lein pom
可以发现新生成的 pom.xml, 添加了新的依赖:
<dependencies>
<dependency>
<groupId>org.clojure</groupId>
<artifactId>clojure</artifactId>
<version>1.8.0</version>
</dependency>
<dependency>
<groupId>org.clojure</groupId>
<artifactId>java.jdbc</artifactId>
<version>0.3.6</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.25</version>
</dependency>
<dependency>
<groupId>korma</groupId>
<artifactId>korma</artifactId>
<version>0.3.0</version>
</dependency>
</dependencies>
使用 Korma 操作数据库十分简单，只需要先引用 Korma。
(ns
^{:author jack}
org.lightsword.course.db
(:use korma.db korma.core) )
定义数据库连接
(defdb korma-db (mysql {
:db "cljweb",
:host "localhost",
:port 3306,
:user "root",
:password "root"
}))
然后定义一下要使用的 entity，也就是表名。
定义 entity
(declare courses)
(defentity courses)现在，就可以对数据库进行操作了。插入一条记录。
执行 insert
(insert courses
(values {:id "007", :name "Clojure Programming", :price 45.8, :online true, :days 30})
)使用 Clojure 内置的 map 类型，十分直观。
查询语句通过 select 宏实现了 SQL DSL 到 Clojure 代码的自然映射。 执行 select
(select courses
(where {:online true})
(order :name :asc)
)这完全得益于 Lisp 的 S 表达式的威力，既不需要直接拼凑 SQL，也不需要重新发明类似 HQL 的语法。
利用 Korma 提供的 sql-only 和 dry-run，可以打印出生成的 SQL 语句.
(ns org.lightsword.course.db
(:use korma.db
korma.core))
(defdb korma-db (mysql {
:db "cljweb",
:host "localhost",
:port 3306,
:user "root",
:password "root"
}))
(declare courses)
(defentity courses)
(defn create-course! [c]
(println "create course:" c)
(insert courses
(values c)
)
)
(defn get-courses []
(select courses
(where {:online true})
(order :name :asc)
)
)
(defn get-all []
(select courses (order :name :asc))
)
(defn init-courses! []
(if (empty? (get-courses))
(let [cs [{:id "s-201", :name "SQL", :price 99.9, :online false, :days 30 },
{:id "s-202", :name "PHP", :price 69.9, :online false, :days 15},
{:id "s-203", :name "F#", :price 80.0, :online false, :days 20}]]
(println "init courses ...")
(dorun (map create-course! cs))
)
)
)
(defn -main [& args]
(init-courses!)
(println (get-courses))
(println (get-all))
)
我们配置一下 main
project.clj
(defproject org.lightsword/clj-web "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url "http://example.com/FIXME"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:main org.lightsword.course.db
:dependencies [[org.clojure/clojure "1.8.0"]
[org.clojure/java.jdbc "0.3.6"]
[mysql/mysql-connector-java "5.1.25"]
[korma "0.3.0"]
])
运行
$ lein run
WARNING: update already refers to: #'clojure.core/update in namespace: korma.core, being replaced by: #'korma.core/update
WARNING: update already refers to: #'clojure.core/update in namespace: org.lightsword.course.db, being replaced by: #'korma.core/update
七月 03, 2016 1:19:55 下午 com.mchange.v2.log.MLog <clinit>
信息: MLog clients using java 1.4+ standard logging.
七月 03, 2016 1:19:55 下午 com.mchange.v2.c3p0.C3P0Registry banner
信息: Initializing c3p0-0.9.1.2 [built 21-May-2007 15:04:56; debug? true; trace: 10]
七月 03, 2016 1:19:55 下午 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
信息: Initializing c3p0 pool... com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 1hge1619h1cm0f3t1s80nsr|d1f74b8, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.mysql.jdbc.Driver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1hge1619h1cm0f3t1s80nsr|d1f74b8, idleConnectionTestPeriod -> 0, initialPoolSize -> 3, jdbcUrl -> jdbc:mysql://localhost:3306/cljweb, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 10800, maxIdleTimeExcessConnections -> 1800, maxPoolSize -> 15, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 3, numHelperThreads -> 3, numThreadsAwaitingCheckoutDefaultUser -> 0, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]
[{:id 007, :name Clojure Programming, :price 45.8, :online true, :days 30}]
[{:id 007, :name Clojure Programming, :price 45.8, :online true, :days 30} {:id s-203, :name F#, :price 80.0, :online false, :days 20} {:id s-202, :name PHP, :price 69.9, :online false, :days 15} {:id s-201, :name SQL, :price 99.9, :online false, :days 30}]
从运行日志我们可以看出, org.clojure/java.jdbc 底层调用的是
driverClass -> com.mysql.jdbc.Driver