# 4.2bean定义中的表达式

可以将SPEL表达式与基于XML或基于注解的配置元数据一起用于定义BeanDefinition实例。在这两种情况下，定义表达式的语法形式都是 #{ \<expression string> }。

## 4.2.1 XML配置

可以使用表达式设置属性或构造函数参数值，如下示例所示：

```
<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>
```

SystemProperties变量是预定义的，因此可以在表达式中使用它，如下示例所示：

```
<bean id="taxCalculator" class="org.spring.samples.TaxCalculator">
    <property name="defaultLocale" value="#{ systemProperties['user.region'] }"/>

    <!-- other properties -->
</bean>
```

注意，在此上下文中，你不必在预定义变量前面加上#符号。

你还可以按名称引用其他bean属性，如下示例所示：

```
<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>

<bean id="shapeGuess" class="org.spring.samples.ShapeGuess">
    <property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/>

    <!-- other properties -->
</bean>
```

## 4.2.2 注解配置

要指定默认值，可以将@value注解放在字段、方法、方法或构造函数参数上。

以下示例设置字段变量的默认值：

```
public static class FieldValueTestBean

    @Value("#{ systemProperties['user.region'] }")
    private String defaultLocale;

    public void setDefaultLocale(String defaultLocale) {
        this.defaultLocale = defaultLocale;
    }

    public String getDefaultLocale() {
        return this.defaultLocale;
    }

}
```

下面的示例显示了等效的但在属性设置器方法上的示例：

```
public static class PropertyValueTestBean

    private String defaultLocale;

    @Value("#{ systemProperties['user.region'] }")
    public void setDefaultLocale(String defaultLocale) {
        this.defaultLocale = defaultLocale;
    }

    public String getDefaultLocale() {
        return this.defaultLocale;
    }

}
```

autowired方法和构造函数也可以使用@value注解，如下示例所示：

```
public class SimpleMovieLister {

    private MovieFinder movieFinder;
    private String defaultLocale;

    @Autowired
    public void configure(MovieFinder movieFinder,
            @Value("#{ systemProperties['user.region'] }") String defaultLocale) {
        this.movieFinder = movieFinder;
        this.defaultLocale = defaultLocale;
    }

    // ...
}
```

```
public class MovieRecommender {

    private String defaultLocale;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao,
            @Value("#{systemProperties['user.country']}") String defaultLocale) {
        this.customerPreferenceDao = customerPreferenceDao;
        this.defaultLocale = defaultLocale;
    }

    // ...
}
```


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