在Kotlin中编写时，会话范围的bean的依赖项设置为null

Question

使用弹簧套2.3.4，kotlin 1.3.72。

我有一个会话范围的bean，Spring正在用null依赖项实例化它，这显然会在执行时产生NPE。我创建了一个示例项目来重现这种行为等效代码在Java中工作UPDATE：如果我只将MessageWriterService转换为Java，则以下操作有效。

这是我的配置类：

@SpringBootApplication
class SessionScopeKotlinApplication {
@Bean
fun messageWriter() = MessageWriter()
@Bean
@SessionScope
fun messageWriterSvc(writer: MessageWriter) = MessageWriterService(writer)
}

MessageWriter和MessageWriterService

class MessageWriter {
fun write() = println("message written!")
}
open class MessageWriterService(private val writer: MessageWriter) {
private var written = false
fun writeIfNotWritten() {
if (!written) {
writer.write()
written = true
} else {
println("** message already written, not writing again **")
}
}
}

然后我有一个控制器，它调用MessageWriterService：

@RestController
class MessageWriterController(private val writerService: MessageWriterService) {
@GetMapping("/write")
fun write() {
writerService.writeIfNotWritten()
}
}

一旦我调用控制器方法，我就会得到下面的stacktrace(缩写(。失败的地方在于对编写器的调用，它实际上是空的。

2020-10-13 07:37:41.919 ERROR 9184 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException: null
at com.example.sessionscopekotlin.MessageWriterService.writeIfNotWritten(SessionScopeKotlinApplication.kt:32) ~[main/:na]
at com.example.sessionscopekotlin.MessageWriterController.write(SessionScopeKotlinApplication.kt:44) ~[main/:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.2.9.RELEASE.jar:5.2.9.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) ~[spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:878) ~[spring-webmvc-5.2.9.RELEASE.jar:5.2.9.RELEASE]

Answer 1

对我来说，Spring似乎无法在这里为您自动连接Writer：

@SpringBootApplication
class SessionScopeKotlinApplication {
@Bean
fun messageWriter() = MessageWriter()
@Bean
@SessionScope
fun messageWriterSvc(writer: MessageWriter) = MessageWriterService(writer)
}

解决方案是这样调用bean函数：

@SpringBootApplication
class SessionScopeKotlinApplication {
@Bean
fun messageWriter() = MessageWriter()
@Bean
@SessionScope
fun messageWriterSvc() = MessageWriterService(messageWriter())
}

如果不希望每次调用都返回相同的Writer，那么也应该使用prototype作用域。

Answer 2

有趣的发现。如果您不想为每个会话创建新的MessageWriter，以下方法可能会有所帮助：

@SpringBootApplication
class SessionScopeKotlinApplication {
@Bean
fun messageWriter() = MessageWriter()
}
@Service
@SessionScope
class MessageWriterService(private val writer: MessageWriter) {
private var written = false
fun writeIfNotWritten() {
if (!written) {
writer.write()
written = true
} else {
println("** message already written, not writing again **")
}
}
}

因此，您基本上从@Bean注释切换到直接的@Service声明。但我可以想象在某些情况下，您希望显式使用@Bean注释。