7494: Unable to view JMC Help Contents #347

cimi · 2022-01-09T15:04:38Z

This PR fixes a problem that makes all JMC help pages fail.

We use the eclipse help plugin, it plugin starts a web server which renders the XML content in the docs plugin via JSP. When the user clicks help in the JMC UI they are sent to the browser, which tries to render one of these JSP pages.

In #306 we introduced new dependencies on multiple Jetty packages so we can run a websocket server inside JMC. After these changes we notice that the help pages fail to render with the following error:

javax.servlet.ServletException: non-HTTP request or response
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:578)
	at org.eclipse.equinox.http.jetty.internal.HttpServerManager$InternalHttpServiceServlet.service(HttpServerManager.java:308)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:764)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:508)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:221)

This is the code that throws the exception:

    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request;
        HttpServletResponse response;

        if (!(req instanceof HttpServletRequest && res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;

        service(request, response);
    }

The help plugin has Jetty as a transitive dependency so it's likely this error is caused by the new servlet-api dependency that we've introduced. Using the debugger, we can set a breakpoint before the exception is thrown and we can inspect req and res to see why the instanceof check fails. The result of this is very strange:

I don't understand why this fails here, since we can see with reflection that req implements the expected javax.servlet.http.HttpServletRequest interface. It's likely this is because we have two servlet-api bundles loaded, org.eclipse.jetty.servlet-api (explicit dependency of flightrecorder.ui) and jakarta.servlet-api (transitive dependency of the help plugin).

I also don't understand if it's possible that instanceof fails because it tries to compare javax.servlet.http.HttpServletRequest loaded from the different bundles, given that the package and interface names are identical.

Regardless of the root cause, removing the explicit dependency on org.eclipse.jetty.servlet-api from the target platform and depending on jakarta.servlet-api in the flightrecorder.ui manifest fixes the issue. Now help pages load successfully and the websocket server works as before.

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Issue

JMC-7494: Unable to view JMC Help Contents

Reviewers

Marcus Hirt (@thegreystone - Reviewer)
Alex Macdonald (@aptmac - Committer)

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Checkout this PR locally:
$ git fetch https://git.openjdk.java.net/jmc pull/347/head:pull/347
$ git checkout pull/347

Update a local copy of the PR:
$ git checkout pull/347
$ git pull https://git.openjdk.java.net/jmc pull/347/head

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Checkout this PR locally:
$ git pr checkout 347

View PR using the GUI difftool:
$ git pr show -t 347

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Download this PR as a diff file:
https://git.openjdk.java.net/jmc/pull/347.diff

We needed a servlet api implementation for the websocket server so we added a dependency on the jetty.servlet-api. We already have jakarta.servlet-api in our dependencies, required by the eclipse help page which also uses jetty to serve dynamic content. We need to avoid loading multiple servlet-apis because it causes errors serving help pages - the server fails to recognise HTTP requests, presumably because of the multiple implementations available.

bridgekeeper · 2022-01-09T15:04:59Z

👋 Welcome back cimi! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

mlbridge · 2022-01-09T15:08:44Z

Webrevs

00: Full (fe4d63f5)

openjdk · 2022-01-09T17:52:12Z

@cimi This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

7494: Unable to view JMC Help Contents

Reviewed-by: hirt, aptmac

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been no new commits pushed to the master branch. If another commit should be pushed before you perform the /integrate command, your PR will be automatically rebased. If you prefer to avoid any potential automatic rebasing, please check the documentation for the /integrate command for further details.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@thegreystone, @aptmac) but any other Committer may sponsor as well.

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cimi · 2022-01-10T10:23:39Z

/integrate

openjdk · 2022-01-10T10:25:08Z

@cimi
Your change (at version fe4d63f) is now ready to be sponsored by a Committer.

cimi · 2022-01-10T10:57:37Z

Since the ticket mentioned the help pages didn't work if we run JMC with JDK 17, I tried it out. I could reproduce it, here is the error:

org.apache.jasper.JasperException: PWC6033: Error in Javac compilation for JSP

PWC6197: An error occurred at line: 41 in the jsp file: /index.jsp
PWC6199: Generated servlet error:
System cannot be resolved


	at org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:129)
	at org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:299)
	at org.apache.jasper.compiler.Compiler.generateClass(Compiler.java:392)
	at org.apache.jasper.compiler.Compiler.compile(Compiler.java:453)
	at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:625)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)

I didn't know how to open the jsp in eclipse so I couldn't play with it there, but looking at the source it seems that because of the System.getProperty call the JSP fails to compile. Searching online for similar problems, I found this issue which suggests upgrading JDT (and specifically the eclipse compiler) might fix it.

aptmac · 2022-01-10T15:39:15Z

I didn't know how to open the jsp in eclipse so I couldn't play with it there, but looking at the source it seems that because of the System.getProperty call the JSP fails to compile. Searching online for similar problems, I found this issue which suggests upgrading JDT (and specifically the eclipse compiler) might fix it.

Guru had opened an Eclipse bug for JDT explaining our issue [0], there hasn't been a lot of comments on it recently though. Interesting that the update to JDT 3.26.0 fixed the issue for open-library, we're currently consuming 3.18 via 2021-06 [1] (and it looks like even through to 2021-12 the same version will be used).

[0] https://bugs.eclipse.org/bugs/show_bug.cgi?id=574937
[1] https://download.eclipse.org/releases/2021-06/202106161001/features/

thegreystone · 2022-01-10T17:26:42Z

/sponsor

openjdk · 2022-01-10T17:27:19Z

Going to push as commit b65e351.

openjdk · 2022-01-10T17:27:41Z

@thegreystone @cimi Pushed as commit b65e351.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

openjdk bot added the rfr label Jan 9, 2022

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cimi deleted the cimi/JMC-7494-fix-help-page branch January 10, 2022 20:06

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7494: Unable to view JMC Help Contents #347

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7494: Unable to view JMC Help Contents #347

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