Must use SE 8 only? java.lang.UnsupportedClassVersionError

[vasigorc]

Hello Colin,

Thank you for your effort to create this library. I am trying to integrate it into an existing GWT application following the small guide on the main page and your sample project. I have however faced this problem when after making the basic set-up and launching the app (I am pasting the stack below). I am trying to run on Oracle 7 JDK and am using GWT 2.7.0.

               Loading inherited module 'com.colinalworth.gwt.websockets.RpcOverWebSockets'
                  [ERROR] Unexpected error while processing XML
java.lang.UnsupportedClassVersionError: com/colinalworth/gwt/websockets/rebind/ServerBuilderGenerator : Unsupported major.minor version 52.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
	at com.google.gwt.dev.cfg.ModuleDefSchema$ClassAttrCvt.convertToArg(ModuleDefSchema.java:902)
	at com.google.gwt.dev.util.xml.HandlerArgs.convertToArg(HandlerArgs.java:64)
	at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:221)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:296)
	at org.apache.xerces.parsers.AbstractSAXParser.startElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:349)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$200(ReflectiveParser.java:70)
	at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:431)
	at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:377)
	at com.google.gwt.dev.cfg.ModuleDefSchema$BodySchema.__inherits_begin(ModuleDefSchema.java:504)
	at sun.reflect.GeneratedMethodAccessor8.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:230)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:296)
	at org.apache.xerces.parsers.AbstractSAXParser.startElement(Unknown Source)
	at org.apache.xerces.parsers.AbstractXMLDocumentParser.emptyElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:349)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$200(ReflectiveParser.java:70)
	at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:431)
	at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:377)
	at com.google.gwt.dev.cfg.ModuleDefSchema$BodySchema.__inherits_begin(ModuleDefSchema.java:504)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:230)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:296)
	at org.apache.xerces.parsers.AbstractSAXParser.startElement(Unknown Source)
	at org.apache.xerces.parsers.AbstractXMLDocumentParser.emptyElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:349)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$200(ReflectiveParser.java:70)
	at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:431)
	at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:377)
	at com.google.gwt.dev.cfg.ModuleDefSchema$BodySchema.__inherits_begin(ModuleDefSchema.java:504)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:230)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:296)
	at org.apache.xerces.parsers.AbstractSAXParser.startElement(Unknown Source)
	at org.apache.xerces.parsers.AbstractXMLDocumentParser.emptyElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
	at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
	at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
	at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
	at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:349)
	at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$200(ReflectiveParser.java:70)
	at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:431)
	at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:377)
	at com.google.gwt.dev.cfg.ModuleDefLoader.load(ModuleDefLoader.java:290)
	at com.google.gwt.dev.cfg.ModuleDefLoader.doLoadModule(ModuleDefLoader.java:229)
	at com.google.gwt.dev.cfg.ModuleDefLoader.loadFromResources(ModuleDefLoader.java:167)
	at com.google.gwt.dev.cfg.ModuleDefLoader.loadFromClassPath(ModuleDefLoader.java:138)
	at com.google.gwt.dev.DevModeBase.loadModule(DevModeBase.java:1005)
	at com.google.gwt.dev.DevMode.loadModule(DevMode.java:695)
	at com.google.gwt.dev.DevMode.doStartup(DevMode.java:566)
	at com.google.gwt.dev.DevModeBase.startUp(DevModeBase.java:1044)
	at com.google.gwt.dev.DevModeBase.run(DevModeBase.java:836)
	at com.google.gwt.dev.DevMode.main(DevMode.java:413)
[ERROR] shell failed in doStartup method

[niloc132]

That's something we can probably change easily, though I'm nearing 0.2 which will drop the generator entirely (so as to be futureproofed with gwt3, and support jvm/teavm/android clients as well, not to mention drop 1500 lines of code as the project is simplified), though I do have to point out that Java 7 is firmly end-of-life'd, and 8 will be too as of January. GWT 2.7 as well is quite old - with JsInterop usage being added to this project as well, you are going to need to be thinking about upgrades soon.

In the short term, can you try checking out the project and just changing to 1.7 in the pom and try rebuilding? I'm afraid I don't even have a machine running with 1.7 any more, but I think that will work as-is and have you on your way. Again though, don't consider this a permanent solution - you will need to update at least GWT to something more recent than four years old, and your JRE as well at some point.

[vasigorc]

I apologize for the delay with answer. Thank you for the answer. I managed to run it under JDK 8 and it worked until this moment:

final MyWebSocketServerBuilder serverBuilder = MyWebSocketServerBuilder.INSTANCE;
serverBuilder.setUrl("ws://"+Window.Location.getHost()+"/lsws");
MyWSServer myWSServer = serverBuilder.start();

in your ServerImpl you are using lambdas, so yes we will need to wait until upgrading to GWT 2.8.0.