Determine Which JDK Version a JAR/Class File Was Compiled With

Today I came across a nasty error which occurred in a deployed Java application only but not during development or integration tests. The error went something like the following:

java.lang.NoSuchMethodError: java.nio.ByteBuffer.rewind()Ljava/nio/ByteBuffer;
at nx.serializerkryo.internal.InternalKryoSerialzer.performToStream(
at nx.serializerkryo.internal.InternalKryoSerialzer.serialize(
at nx.serializerkryo.internal.InternalKryoSerialzer.serialize(
at nx.persistence.jre.internal.OptimizedPersistedNodeSerializer.serialize(
at nx.persistence.jre.internal.OptimizedPe<span 				data-mce-type="bookmark" 				id="mce_SELREST_end" 				data-mce-style="overflow:hidden;line-height:0" 				style="overflow:hidden;line-height:0" 			></span>rsistedNodeSerializer.serialize(

Now I had a feeling that this had something to do with me trying to be ahead of the curve and use a Java 9 JDK to compile the application. In order to debug this, I had to confirm which with JDK the classes I was using were compiled with. Thankfully I found a handy thread of StackOverflow.

Unfortunately, it wasn’t immediately obvious to me which solution listed there would work best, so I decided to provide the solution here in a more condensed form. Simply use the following command:

javap -v [path to your class file]

The output will then contain the following line (towards the top of the file):

public class ...
minor version: 0
major version: 50

The major version and minor version indicates which version of Java the class was compiled with. The following contains a list of which Java versions which major versions relate to.

Java SE 9 = 53 (0x35 hex),
Java SE 8 = 52 (0x34 hex),
Java SE 7 = 51 (0x33 hex),
Java SE 6.0 = 50 (0x32 hex),
Java SE 5.0 = 49 (0x31 hex),
JDK 1.4 = 48 (0x30 hex),
JDK 1.3 = 47 (0x2F hex),
JDK 1.2 = 46 (0x2E hex),
JDK 1.1 = 45 (0x2D hex).

Interestingly my files were apparently compiled for Java 6 (Maven compiler plugin was responsible). The problem was that the files were compiled with JDK 9 (though they were compiled for 1.6). Downgrading the JDK used to do the compilation to JDK8 fixed the problem.

A Handy Reference of Maven Parameters

I cannot count the times I have looked up the following through Google. Thus I decided to put together a few handy parameters (or properties or whatever is the correct term) for Maven builds.

All the below are given with the goal install but they can safely be used with any other Maven goal as well.

Skip Tests

mvn install -DskipTests

Build Only From Specified Project

This is only relevant in a multi pom.

mvn install -rf :[artifactId]

Don’t Compile JavaDoc


Don’t Compile GWT



Install Latest JDK on Linux Server

To install the Oracle JDK on a Linus server is often a tricky proposition. For one, the download page requires to confirm a prompt and only unlocks the download link after this prompt has been confirmed (via a cookie, I think). This makes it difficult to download the binary in the first place!

Thankfully, MaxdSre has created the following handy script to download and extract the JDK:

If you run this script, you are presented with a prompt as follows:


Just select the version you require, and the script will download and install the Oracle JDK.

Finally, you might have existing JDK versions installed on your machine which are managed using alternatives. For reference how to point your ‘java’ command to the new installation, please see this article.


Library for Parsing multipart File Upload with Java

One of the most convinient ways to upload files from the Web Browser to the server is by using file inputs in HTML forms.

Many web servers come with preconfigured modules for parsing this data on the server-side. However, sometimes, your HTTP server of choice might not offer such a module and you are left with the task of parsing the data the browser submits to the server yourself.

I specifically encountered this problem when working with a Netty-based server.

The form will most likely submit the files to your server as part of a multipart/form-data request. These are not that straightforward to parse. Thankfully, there is the library Apache Commons FileUpload which can be used for this purpose.

Unfortunately, processing some arbitrary binary data with this library is not very straightforward. This has motivated me to write a small library – delight-fileupload –  which wraps Commons FileUpload and makes parsing multipart form data a breeze. (This library is part of the Java Delight Suite).

Just include the library and let it parse your data as follows:

FileItemIterator iterator = FileUpload.parse(data, contentType);

Where data is a binary array of the data you received from the client and contentType is the content type send via HTTP header.

Then you can iterate through all the files submitted in the form as follows:

while (iter.hasNext()) {
 FileItemStream item =;
 if (item.isFormField()) {
   ... some fields in the form
 } else {
   InputStream stream = item.openStream();
   // work with uploaded file data by processing stream ...

You can find the library on GitHub. It is on Maven Central. Just add the following dependency to your Java, Scala etc. application and you are good to go:


You can also check for the newest version on the JCenter repostiory.

I hope this is helpful. If you have any comments or suggestions, leave a comment here or raise an issue on the javadelight-fileupload GitHub project.



Java: Find all instances of System.out.println()

A good Java application should print only the absolute necessary to standard out. Not to do so can mean a serious hit in performance and can make it difficult to debug issues.

Unfortunately either we ourselves or our colleagues put System.out.println() statements in the code and then forget to remove them.

Here are two solutions how to find all those nasty statements.

Solution 1: Do a Full Text Search

Use your IDE and search for all occurrences of the string “System.out.println()” in your code. In eclipse this can be done through Menu / Search / File …

Make sure to use *.java for the file name pattern.


Solution 2: Put a Breakpoint in PrintStream

The above solution might not be practical if there are many System.out statements in the code which are legitimately there. In that case, you can also put a break point into the Java standard class PrintStream.

In eclipse, open the search for class dialog (Ctrl+Shift+t) and type in the class name PrintStream from the package


In this class, find the print(x) method and add a breakpoint:


Then run your program in debug mode. You can then find the places where System.out is called from the stack trace.


Understanding Creation of GWT Serialization Policy Files (.gwt.rpc)

Today I deep-dived a bit into how GWT creates and reads Serialization Policy files. These are the .gwt.rpc files which are generated beside the JavaScript files for GWT modules.

Chiefly, I learned two things:

  • The .gwt.rpc files are only used by the server. The client never reads them.
  • The .gwt.rpc files are generated in the ProxyCreator class.

I have listed some further classes and links below.

GWT Framework Classes

ProxyCreator: Creates the .gwt.rpc file

RemoteServiceProxy: Manages calls to services

ClientSerializationStreamWriter: Write serialization on client for server

ClientSerializationStreamReader: Read responses from server on client

Serializer: Interface for serialization contract for class


The GWT RPC Wire Protocol


Run GWT Generated Code in Nashorn

GWT is a very useful tool to compile Java code into JavaScript.

Nashorn is Oracle’s new JavaScript implementation which runs JavaScript scripts in Java.

In order to run JavaScript server-side code within a Java server I now want to make an extensive GWT library available to this JavaScript code.

For this I need to load code generated in GWT into a Nashorn engine.

This is not very easy since the load process of GWT libraries includes various ‘hacks’ which involve the DOM.

I don’t know yet how I am going to do this exactly. I found the gwt-node project. This project is meant to run GWT code on Node.js.

I think by working with the custom linker developed there ( I might be able work something out.

Sandboxing JavaScript in Java App – Link Collection

The JVM is by design an insecure environment and it is generally difficult to run untrusted code in a sandboxed environment.

However, it seems that is relatively easy to sandbox JavaScript code running in Oracle Nashorn. The instructions are here. Strangely, this was not easy to find through a Google search.

Below I have listed some further sources on Sandboxing JavaScript and Java code. Although there is plenty of material on Rhino, I would not recommend using this engine. I think Nashorn has been designed with support for Sandboxed code in mind from the very beginning while in Rhino the functionality feels kind of bolted on.

UPDATE I have implemented two little libraries which takes care of the grunt work of sandboxing Nashorn and Rhino code in Java:

Nashorn Sandbox (on GitHub)

Rhino Sandbox (on Github)

Sandboxing JavaScript


Restricting Script Access to Specified Java Classes: From the Oracle Nashorn docs. Shows how to restrict access to specific Java classes.


Class ContextFactory: Useful for monitoring and setting restrictions on Rhino code.

Method initSafeStandardObjects: Useful for creating sandboxed Rhino code.

Rhino Sandbox: A small library for sandboxing JavaScript code running in Rhino.

Sandboxing Rhino in Java: Blog post

Securing Rhino in Java6: Blog post


Sandboxing JavaScript Execution in Java: Blog post

Sandboxing Java

Example Code Monitoring Threads: Example code how thread CPU usage can be monitored.

The Java Sandbox: A library for sandboxing any Java code. Might be useful to sandbox the Java code with runs the script.

Use Signed SSL Certificate with Java

It seems to me like every time I have to do something that has to do with SSL certificates – be they self-signed or signed by a certificate authority – things do not go smoothly. I only do this from time to time, so I am by no means an expert; but I do believe my difficulties result to some degree from the intrinsic complexities within SSL and the systems which support it.

I have created another guide which walks step by step through the process of configuring a Java key store with a signed SSL certificate. If you do want this to succeed, note that you have to follow every step precisely. Even minor omissions can lead to errors (believe me, I’ve tried it myself).

  • Download Portecle (from here) and Unzip it
  • Start portecle.jar by double clicking it
  • Go to File / New Keystore

  • Select JKS and click [OK]
  • Go to Tools / Generate Key Pair

  • Select Algorithm RSA and Key size 2048
  • Increase validity from the default 356 to 1000 or more days

  • In Common Name provide the domain or subdomain of the domain you want to protect
  • Provide some input for all other fields – do not leave any empty
  • Provide a password and remember it (This can be the same password as for the the whole store)
  • Provide an alias – best the name of your domain

  • You should see the following:

  • Right click the key pair you have create and select ‘Generate Certification Request’

  • Portecle will generate file ‘XYZ.csr‘ for you.
  • Provide the contents of this file to the SSL provider of your choice (see a brief comparison here – I’ve had good experiences with RapidSSL certificates from GoGetSSL).
  • Your SSL provider should supply you with an SSL certificate. This file should end with ‘.crt‘. Download it.
  • Go back to Portecle and right click your key pair again. Select ‘Import CA Reply’.

  • Import the .crt file you got from your SSL provider.
  • If this does not work, first proceed to import the certificates as listed in the next steps, then try again to import the CA Reply.
  • You can import the ROOT certificate of your SSL provider just in case.
  • Also, your SSL provider will supply you with an intermediate and server certificate. You can import these into your keystore as well.
  • Note that when importing the ROOT certificate of your provider, you might get a warning that no trust chain can be established to the certificate. However, when importing the intermediate and server certificates AFTER importing the root certificate, there should be no warning that no chain can be established.
  • Your keystore should look something like this now:

  • Now go to File / Save Keystore
  • Provide the same password you used before.

Now you can use the created key store in Java servers. For an easy way how to use a keystore with Java, check out step 7 in this post.

Using RapidSSL Certificate from GoGetSSL for Java Server

IMPORTANT: I found it a lot easier and less error prone to use the GUI tool Portecle to go about generating a SSL certificate/key. You can find my instructions to do so in another post.

The following steps show how a RapidSSL certificate obtained through GoGetSSL can be used to secure a Java server.

Step 1: Purchase Certificate

Go to gogetssl and purchase a Standard RapidSSL certificate (should be around $5 / year).

Step 2: Create Keystore


keytool -keysize 2048 -genkey -alias tomcat -keyalg RSA -keystore server.keystore

When asked for ‘What is your first and last name?’ enter the domain of your server (can also be a subdomain).

Press ENTER when prompted for ‘Enter key password for <tomcat>’

Step 3: Create CSR 


keytool -certreq -keyalg RSA -alias tomcat -file server.csr -keystore server.keystore

Open the file server.csr and copy its contents into the clipboard.

Step 4: Upload CSR to GoGetSSL

Login to GoGetSSL and select ‘Manage SSL Certificates’ / All.

Next to the certificate you have just purchased should be a [Generate] button. Click it.

Choose ‘Order Type’: ‘New Order’

Choose ‘Web Server Software’: ‘Jakart-Tomcat’

Paste the CSR you copied from server.csr.

Choose signature algorithm SHA2.

Click [Validate CSR]

Step 5: Perform Email Validation and Give Your Details

Specify an email address to which the validation email should be sent and click [Next Step].

Also give your details and confirm the RapidSSL terms and conditions.

Note: Now wait a few minutes until you get the email and confirm it when you got it.

Step 6: Import RapidSSL Certificates Into Keystore

You will receive an email with the certificate for the server and the intermediate certificate from RapidSSL.

You’ll need to add both to your keystore.

First the intermediate certificate:

Get it from the email and paste it into a file ‘intermediate.crt’ and put it into the same folder as you keystore. Then run:

keytool -import -trustcacerts -alias intermediate -file intermediate.crt -keystore server.keystore

You should get a message ‘Certificate was added to keystore’

Then the server certificate:

Get it from the email and paste it into a file ‘server.crt’ and put it into the same folder as you keystore. Then run:

keytool -import -trustcacerts -alias server -file server.crt -keystore server.keystore

You should again get a message ‘Certificate was added to keystore’.

Now you can use server.keystore to secure your Java Webserver with SSL.