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I've upgrade my Java JDK installation to a later minor release version (e.g. 1.8.131 to 1.8.144).
Should I recompile project source code to get any security benefits from bytecode generation or should I just run the old bytecode on the updated JRE?
Any real world examples welcome as I will need to justify this to the project team.
You are mistaken.
The java compiler doesn't do much, besides turning Java source into bytecode. There are few special optimisations. And no "security" related things at all.
And keep in mind: all invokations of system methods that your code is doing - will be going to Java8 classes on a Java8 VM.
Thus there is no pressing reason to update "everything".
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I have couple of projects using JDK 1.8 in production. However Java is paid from January 2019 onwards. Is my problem solved if I migrate my projects to OpenJDK 1.8 because it's open source?
As far as I know, it concerns only the Oracle Java APIs.
OpenJDK and OpenJFX are largely independent (besides getting contributions from Oracle).
As general Oracle Java was a bit smarter than the OpenJDK (on Linux), the entire outcries are a storm in a glass of water: it makes a (ill guided) sense to let Oracle be paid for its development effort.
The thing to do, is trying out the OpenJDK, and taking notion of com.sun classes.
You might need to look for new versions of your typical server or framework, that might rely on com.sun.* classes.
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When I was first learning java from a crash course, we were advised by the teacher to change the jdk after long use.
While she couldn't provide any scientific explanation to this, even I find it necessary to change the jdk at points of time. In the computer at my home, sometimes it happens that a program that I've written, is syntactically, semantically and logically correct, but both BlueJ and NetBeans give unexplained compilation or run-time error. Both BlueJ and NetBeans run on the same jdk.
If I download a new jdk package and install it, removing the previous one, it solves the problem.
Can anyone explain this?
New versions of the JDK can run old code, but the opposite is not necessarily true: Recent applications will take advantage of recent features of the JDK, and will not be compatible with older versions.
Note that sometimes, applications built with some JDK may not run with newer versions. For instance, JDK 7 has removed some deprecated classes from JDK1.6.
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I'm writing an application in java. In order to work my code properly, I want to check whether several software are installed. (This should be done in the java code itself.)
How can I do that?
This is a Linux system.
There are similar questions, but not exactly what I want.
Check if a program is installed on a linux machine from a java applet
There is no universal way of doing it.
You can use one of following approaches
Use java bindings for particular package manager for your platform if they exist.
Use one of the system tools like locate, whereis, etc. (which you didn't like)
Implement your own installed package detection, according to your requirements
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Problem: I can not understand the meaning of the question and another doubt which is generated from the above problem is that all the java software which i am using are come with executable setup so i am little bit confused from this.
Thank You
Because exe's have to be compiled for specific environments.
Oracle compiles their runtime (JRE) for different operating systems, which interprets your Java file anywhere that has a JRE installed. You can however make an installer for it:
Create Windows Installer for Java Programs
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I am using applet code in my application. When I am running that applet it is aksing me to install java 7 version.
Is it mandatory to install java7 version?
If it is mandatory how the applets were run in java6 version previously?
I am confused.
Is it mandatory to install java7 version.
Basically, yes. And you should do it anyway, since Java 6 is no longer getting security patches.
If it is mandatory how the applets were run in java6 version previously.
The author has (presumably) changed something. He/she might have started using Java 7 language constructs or Java 7 specific library APIs. Or he/she might have just changed the JNLP configurations to force you to upgrade for your own good.