如何在 spring EL 中将 null 作为可选值传递？

Question

正如这篇文章中所建议的，我正在尝试从属性文件中读取列表strings。

@Value("#{'${app.messages}'.split(',')}")
    private List<String> messages;

我收到以下错误：

Error creating bean with name 'MyController': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.app.controller.HealthManager com.app.MyController.Manager; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'healthManager': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.List com.app.Manager.messages; nested exception is java.lang.IllegalArgumentException: Could not resolve placeholder 'health.messages' in string value "#{'${health.messages}'.split(',')}"
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:292)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1185)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:304)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:300)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:195)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:700)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:760)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:482)
    at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:403)
    at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:306)
    at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4937)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5434)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:633)
    at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1113)
    at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1671)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

我也尝试了以下两种方法。我仍然收到错误。

@Value("#{'${app.messages}'.split(','): {null}}")
    private List<String> messages;


@Value("#{'${app.messages}'.split(','): #{null}}")
    private List<String> messages;

我假设传递 null 可能会解决这个问题，因为app.messages在我的情况下是完全可选的。

Answer 1

尝试

@Value("#{'${app.messages:}'.length() > 0 ? '${app.messages:}'.split(',') : null}")

末尾的:表示如果未找到该属性，则使用零长度字符串。