有没有在 SpringXML 中指定默认属性值的方法？

我们在 Spring 配置(详情请浏览此网页)中使用 PropertyPlaceholderConfigrer来使用 java 属性

例如:

<foo name="port">
<value>${my.server.port}</value>
</foo>

我们希望添加一个额外的属性，但是要有一个分布式系统，其中现有的实例都可以使用默认值。有没有一种方法可以避免更新所有的属性文件，在 Spring 配置中指定一个默认值，以便在没有定义重写属性值时使用？

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最佳答案

Are you looking for the PropertyOverrideConfigurer documented here

http://static.springsource.org/spring/docs/2.5.x/reference/beans.html#beans-factory-overrideconfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, is similar to the PropertyPlaceholderConfigurer, but in contrast to the latter, the original definitions can have default values or no values at all for bean properties. If an overriding Properties file does not have an entry for a certain bean property, the default context definition is used.

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Spring 3 supports ${my.server.port:defaultValue} syntax.

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http://thiamteck.blogspot.com/2008/04/spring-propertyplaceholderconfigurer.html points out that "local properties" defined on the bean itself will be considered defaults to be overridden by values read from files:

<bean id="propertyConfigurer"class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location"><value>my_config.properties</value></property>
<property name="properties">
<props>
<prop key="entry.1">123</prop>
</props>
</property>
</bean>
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<foo name="port">
<value>${my.server.port:8088}</value>
</foo>

should work for you to have 8088 as default port

See also: http://blog.callistaenterprise.se/2011/11/17/configure-your-spring-web-application/

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There is a little known feature, which makes this even better. You can use a configurable default value instead of a hard-coded one, here is an example:

config.properties:

timeout.default=30
timeout.myBean=60

context.xml:

<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location">
<value>config.properties</value>
</property>
</bean>


<bean id="myBean" class="Test">
<property name="timeout" value="${timeout.myBean:${timeout.default}}" />
</bean>

To use the default while still being able to easily override later, do this in config.properties:

timeout.myBean = ${timeout.default}
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The default value can be followed with a : after the property key, e.g.

<property name="port" value="${my.server.port:8080}" />

Or in java code: 

@Value("${my.server.port:8080}")
private String myServerPort;

See:

BTW, the Elvis Operator is only available within Spring Expression Language (SpEL),
e.g.: https://stackoverflow.com/a/37706167/537554

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Also i find another solution which work for me. In our legacy spring project we use this method for give our users possibilities to use this own configurations:

<bean id="appUserProperties" class="org.springframework.beans.factory.config.PropertiesFactoryBean">
<property name="ignoreResourceNotFound" value="false"/>
<property name="locations">
<list>
<value>file:./conf/user.properties</value>
</list>
</property>
</bean>

And in our code to access this properties need write something like that:

@Value("#{appUserProperties.userProperty}")
private String userProperty

And if a situation arises when you need to add a new property but right now you don't want to add it in production user config it very fast become a hell when you need to patch all your test contexts or your application will be fail on startup.

To handle this problem you can use the next syntax to add a default value:

@Value("#{appUserProperties.get('userProperty')?:'default value'}")
private String userProperty

It was a real discovery for me.


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