Spring中@Value的三种使用方式

Spring中@Value的三种使用方式

文章目录

• Spring中@Value的三种使用方式

@Value 的作用是什么？我们可以从 Spring官方文档 里面找到这样的一句话：

image.png

接下来我们看下 @Value 注入外部属性的三种方式：

1. 注入普通属性

@Value 注解可以注入一些字段的普通属性，并且会自动进行 类型转换

image.png

栗子：

<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Repository
public class ConnectionPool {
@Value("jdbc:mysql://localhost:3306/test")
private String url;
@Value("com.mysql.jdbc.Driver")
private String driveName;
@Value("Scott")
private String userName;
@Value("10")
private int no;
//重写下toString方法
}</pre>

定义配置类，用于扫描bean

<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Configuration
@ComponentScan("com.fx.dao")
public class myConfig2 { }</pre>

测试一下：

<pre class="prettyprint hljs cpp" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Test
public void test01(){
ApplicationContext context = new AnnotationConfigApplicationContext(myConfig2.class);
ConnectionPool connectionPool = context.getBean("connectionPool", ConnectionPool.class);
System.out.println(connectionPool.toString());
}</pre>

输出：

<pre class="prettyprint hljs xquery" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">ConnectionPool{url='jdbc:mysql://localhost:3306/test', driveName='com.mysql.jdbc.Driver', userName='Scott',no=10}</pre>

2.注入配置文件

可以通过 @Value("${}") 来注入配置文件里面的信息

栗子：

<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Repository
public class ConnectionPool {
@Value("jdbc:mysql://localhost:3306/test")
private String url;
@Value("com.mysql.jdbc.Driver")
private String driveName;
@Value("Scott")
private String userName;
@Value("10")
private int no;
@Value("${mysql.pwd}")
//重写下toString方法
}</pre>

修改下配置类，扫描配置文件：

<pre class="prettyprint hljs less" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Configuration
@ComponentScan("com.fx.dao")
@PropertySource({"classpath:db.properties"})
public class myConfig2 { }</pre>

db.properties 配置文件放在 resources 目录下，使用 @PropertySource 注解扫描的 classpath 就是此路径

<pre class="hljs vim" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">mysql.pwd=123456</pre>

测试代码和上面一样我们输出下结果：

<pre class="prettyprint hljs vim" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">ConnectionPool{url='jdbc:mysql://localhost:3306/test', driveName='com.mysql.jdbc.Driver', userName='Scott', no=10, pwd='123456'}</pre>

3. 注入表达式并运算

image.png

SpEl (Spring Expression Language )，是 Spring 的表达式语言，很多框架中都有自己的 El表达式 ，Spring中El表达式支持的功能有很多，在 官方文档 里面有详细的介绍和用例。

栗子：

例如我想动态获得运行机器的核心数作为参数注入

<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Repository
public class ConnectionPool {
@Value("jdbc:mysql://localhost:3306/test")
private String url;
@Value("com.mysql.jdbc.Driver")
private String driveName;
@Value("Scott")
private String userName;
@Value("10")
private int no;
@Value("${mysql.pwd}")
private String pwd;
@Value("#{T(java.lang.Runtime).getRuntime().availableProcessors()}")
private int minCons;
@Value("#{T(java.lang.Runtime).getRuntime().availableProcessors() * 2}")
private int maxCons;
//toString方法
}</pre>

测试用例和配置类不变，我们输出一下结果:

<pre class="prettyprint hljs vim" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">ConnectionPool{url='jdbc:mysql://localhost:3306/test', driveName='com.mysql.jdbc.Driver', userName='Scott', no=10, pwd='123456', minCons=8, maxCons=16}</pre>

可以看到 SpEl 里执行了我们想要运行的代码

当然我们还可以用 @Value 执行更多更厉害的EL表达式， 官方文档

image.png

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