Spring SPEL：setRemoteDirecotoryExpression问题

Question

我正在尝试使用Java Spring的SPEL语言为我的SFTP服务器设置远程目录。基本上，我试图将两个字符串一起添加为我的远程目录路径，但是由于某种原因，我仍然试图弄清楚，它不起作用。这是我的工作：

ExpressionParser EXPRESSION_PARSER = new SpelExpressionParser();
Expression exp = EXPRESSION_PARSER.parseExpression("headers['path']"); //which contains the 2nd part of the path to my dir
String fullFilePath = this.remoteDir; //which has the 1st part of the path to my directory
SftpMessageHandler handler = new SftpMessageHandler(sftpSessionFactory());
handler.setRemoteDirectoryExpressionString(this.remoteDir.concat("/").concat(exp.getValue().toString()));
handler.setAutoCreateDirectory(true);

exp.getValue()方法似乎有问题，因为每当未注释该行时，我都会收到以下异常：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'SFTPConfig' defined in file [/Users/psdev/PrediSurge/backend/file-service/target/classes/com/predisurge/planopsuite/microservices/fileservice/sftp/SFTPConfig.class]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'handler' defined in class path resource [com/predisurge/planopsuite/microservices/fileservice/sftp/SFTPConfig.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.messaging.MessageHandler]: Circular reference involving containing bean 'SFTPConfig' - consider declaring the factory method as static for independence from its containing instance. Factory method 'handler' threw exception; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headers' cannot be found on null
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:584) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:498) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:846) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:863) ~[spring-context-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:546) ~[spring-context-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) [spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) [spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.2.RELEASE.jar:2.1.2.RELEASE]
    at com.predisurge.planopsuite.microservices.fileservice.FileServiceApplication.main(FileServiceApplication.java:26) [classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_241]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_241]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_241]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_241]
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.1.2.RELEASE.jar:2.1.2.RELEASE]
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'handler' defined in class path resource [com/predisurge/planopsuite/microservices/fileservice/sftp/SFTPConfig.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.messaging.MessageHandler]: Circular reference involving containing bean 'SFTPConfig' - consider declaring the factory method as static for independence from its containing instance. Factory method 'handler' threw exception; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headers' cannot be found on null
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:627) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:456) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1288) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1127) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:498) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.integration.config.annotation.AbstractMethodAnnotationPostProcessor.resolveTargetBeanFromMethodWithBeanAnnotation(AbstractMethodAnnotationPostProcessor.java:464) ~[spring-integration-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
    at org.springframework.integration.config.annotation.AbstractMethodAnnotationPostProcessor.postProcess(AbstractMethodAnnotationPostProcessor.java:146) ~[spring-integration-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
    at org.springframework.integration.config.annotation.MessagingAnnotationPostProcessor.processAnnotationTypeOnMethod(MessagingAnnotationPostProcessor.java:190) ~[spring-integration-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
    at org.springframework.integration.config.annotation.MessagingAnnotationPostProcessor.lambda$postProcessAfterInitialization$0(MessagingAnnotationPostProcessor.java:163) ~[spring-integration-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
    at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:589) ~[spring-core-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.integration.config.annotation.MessagingAnnotationPostProcessor.postProcessAfterInitialization(MessagingAnnotationPostProcessor.java:144) ~[spring-integration-core-5.1.2.RELEASE.jar:5.1.2.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:434) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1749) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:576) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    ... 20 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.messaging.MessageHandler]: Circular reference involving containing bean 'SFTPConfig' - consider declaring the factory method as static for independence from its containing instance. Factory method 'handler' threw exception; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headers' cannot be found on null
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:622) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    ... 38 common frames omitted
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headers' cannot be found on null
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:213) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:104) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:91) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.ast.CompoundExpression.getValueRef(CompoundExpression.java:53) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:89) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:109) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:138) ~[spring-expression-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at com.predisurge.planopsuite.microservices.fileservice.sftp.SFTPConfig.handler(SFTPConfig.java:101) ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_241]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_241]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_241]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_241]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    ... 39 common frames omitted

我不是Java Spring的新手，但是我以前从未使用过SPEL，所以我不确定什么地方出了问题。任何帮助，将不胜感激。

Answer 1

EL表达式“ headers ['path']”可能是指包含键“ path”的映射。默认情况下，spring表达式解析器不包含有关外部变量的信息。您应该创建StandardEvaluationContext的实例，在获取表达式值时在其中注册变量“ headers”并使用此上下文：

ExpressionParser EXPRESSION_PARSER = new SpelExpressionParser();

// create context and register variable "headers"
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("headers",headers);

Expression exp = EXPRESSION_PARSER.parseExpression("headers['path']"); 
String fullFilePath = this.remoteDir; 
SftpMessageHandler handler = new SftpMessageHandler(sftpSessionFactory());

// use context, when getting evaluated value
 handler.setRemoteDirectoryExpressionString(this.remoteDir.concat("/").concat(exp.getValue(context).toString()));
        handler.setAutoCreateDirectory(true);

这里是在弹簧EL中使用变量的reference。