字节码知道Java版本吗

Question

抱歉，如果这是一个愚蠢的问题，我还没有找到直接的答案。

我已经编译了一个用Java 9功能编写的程序，并生成了一个.jar。

该程序的本质是我需要它同时在两台计算机上运行，​​一台正在运行Java 8，一台正在运行Java9。我的程序无法正常工作，特别是jdk.incubator.httpclient模块无法从一台计算机发送HttpRequest。机到另一个。

不能将JDK8计算机升级到9。

我的问题是，一旦全部变成字节码，这有关系吗？ Java 9生产的jar中是否会有Java 8 JVM无法理解的字节码？

可能值得注意的是，我必须使用java -jar --add-modules=jdk.incubator.httpclient运行jar才能使用新的Java 9孵化器模块httpclient

我知道一个聪明的解决方案是确保双方的防火墙都允许这两者对话，但是我也想知道上述答案！

提前致谢。

这是堆栈跟踪：

2018-01-09 01:15:57.410  INFO 4656 --- [nio-8080-exec-2] c.e.s.controller.MessageController       : [TO http://192.168.1.64:8080/send] Hello Bob it is Jim!
2018-01-09 01:18:08.551 ERROR 4656 --- [nio-8080-exec-2] c.e.s.service.MessagingService           : send: unable to send request 

java.net.ConnectException: Connection timed out
    at java.base/sun.nio.ch.Net.connect0(Native Method) ~[na:na]
    at java.base/sun.nio.ch.Net.connect(Net.java:454) ~[na:na]
    at java.base/sun.nio.ch.Net.connect(Net.java:446) ~[na:na]
    at java.base/sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:648) ~[na:na]
    at jdk.incubator.httpclient/jdk.incubator.http.PlainHttpConnection.connect(PlainHttpConnection.java:142) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.Http1Exchange.sendHeadersOnly(Http1Exchange.java:136) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.Exchange.responseImpl0(Exchange.java:298) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.Exchange.responseImpl(Exchange.java:245) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.Exchange.response(Exchange.java:121) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.MultiExchange.response(MultiExchange.java:154) ~[jdk.incubator.httpclient:na]
    at jdk.incubator.httpclient/jdk.incubator.http.HttpClientImpl.send(HttpClientImpl.java:234) ~[jdk.incubator.httpclient:na]
    at com.example.simplemessenger/com.example.simplemessenger.service.MessagingService.send(MessagingService.java:30) ~[classes/:na]
    at com.example.simplemessenger/com.example.simplemessenger.controller.MessageController.sendMessage(MessageController.java:71) [classes/: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:564) ~[na:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/javax.servlet.http.HttpServlet.service(HttpServlet.java:661) [tomcat-embed-core-8.5.23.jar:na]
    at spring.webmvc@4.3.13.RELEASE/org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) [spring-webmvc-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.websocket@8.5.23/org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:108) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) [spring-web-4.3.13.RELEASE.jar:na]
    at spring.web@4.3.13.RELEASE/org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.13.RELEASE.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.23.jar:na]
    at tomcat.embed.core@8.5.23/org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.23.jar:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) [na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) [na:na]
    at tomcat.embed.core@8.5.23/org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.23.jar:na]
    at java.base/java.lang.Thread.run(Thread.java:844) [na:na]

2018-01-09 01:18:08.552  INFO 4656 --- [nio-8080-exec-2] c.e.s.controller.MessageController       : failed to send

Answer 1

答案是肯定的。 在Java 8上，可以使用工具javap看到这一点

$ javap -v Hello.class | grep "version"
  minor version: 0
  major version: 52

在Java 9上，您会看到

  minor version: 0
  major version: 53

但是，您可以使用编译器标志-target和-source来使Java 9编译器生成Java 8兼容代码。

$ javac -source 1.8 -target 1.8 Hello.java

但是，新的孵化器可能尚未反向移植到Java 1.8。 在这种情况下，我建议最好的选择是使用apache HttpComponents-两者都应该起作用。

Answer 2

这个问题有两个不同的组成部分。 第一个问题是，正如其他人已经提到的那样，类文件包含版本标记，并且JVM将拒绝运行任何版本号过高的类文件，这意味着Java 8 JVM将拒绝包含Java 9版本代码的类文件（反向，但完全可以）。

更加有趣的问题是，如果您手动将版本代码编辑为Java 8，则代码是否可以工作。在这种情况下，这取决于代码。 在字节码级别，Java 9中唯一值得注意的更改与模块和软件包有关。 如果您不使用模块，则您的代码仍应在Java 8上运行，前提是它所依赖的所有库在Java 8中也可用。

Java 9还为模块添加了几个属性，但是对于运行时行为而言，这些属性并不重要，除非使用反射，并且JVM会忽略它无法识别的属性，因此不会阻止您的代码在Java 8下运行。

所以tl; dr; 如果您不使用模块，则假设您修复了版本字节，则您的代码应在Java 8下工作。 但是，当您只需将-source和-target标志传递给javac来完成相同的事情时，这将带来很多麻烦。

Answer 3

我的问题是，一旦全部变成字节码，这有关系吗？ Java 9生产的jar中是否会有Java 8 JVM无法理解的字节码？

是的，运行Java 8的JVM将无法运行Java 9代码。 否则，您将需要使用Java 8进行编译。