What does it mean that Java applet runs in web browser? Does it share the same address space as browser? Isn't it created as single process?
Since Java SE 6u10:
The new way of executing applets has architectural similarities to Java Web Start technology, but tighter browser integration. Applets no longer execute in a Java Virtual Machine (JVM) inside the web browser. Instead, a separate JVM machine process is launched to execute applets. By default, only one JVM machine is launched, but you have the opportunity to launch more than one JVM machine, and you get support per-applet command-line arguments, so you can affect heap size or other requests.
Most modern browsers support plugins, for instance through the NPAPI.
This allows the Java runtime to run inside of the browser process, as a plugin, executing the Java applet.
Whether or not it runs in the same process or not is up to the browser itself; Chrome, for instance, runs every plugin in a separate process, while other browsers only use one process.
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I'm working on a application based on a simple monolithic architecture :rest API backend with spring-boot + frontend with a Js framework.
I have been reading about the headless mode when running java apps and I would like to know if running the embedded tomcat with my spring-boot application in headless mode is a good idea.
Generally you don't need to specify -Djava.awt.headless=true because since around Java 1.8 the JRE determines itself whether a graphics environment is available or not and switches into headless mode if no graphics environment is available.
Relevant source in Java 11: https://github.com/openjdk/jdk11/blob/master/src/java.desktop/share/classes/java/awt/GraphicsEnvironment.java#L166
if you are running a Unix-like operating system and the DISPLAY environment variable is not set the headless is true
on Windows it is assumed that a graphics device is always present and therefore headless is false
In general, running an application in a console requires some sort of terminal to write to.
In Unix-based systems, these terminals are referred to as tty.
This implies the following:
Whenever you open a connection to your server via SSH, you get a tty. Your non-headless application will run in that tty until the tty is closed. To avoid that, you'd need something like screen.
For Windows systems: If you close the cmd window of the console/sign off the user, the application server will also stop running.
That's why productive installations are usually headless and service based.
So much for the background, the official docs actually state the following:
In addition to running Spring Boot applications by using java -jar, it
is also possible to make fully executable applications for Unix
systems. A fully executable jar can be executed like any other
executable binary or it can be registered with init.d or systemd. This
helps when installing and managing Spring Boot applications in common
production environments.
This is described here.
I am troubleshooting an issue with a Java Desktop App running on a Citrix server which allows multiple users to run the app at the same time in their own Shared Desktop image.
When the application fails to find a class file throwing NoClassDefFoundError as expected and makes one user session to fail obviously. Then if another user connects to the same Citrix server via Shared Desktop image starting a separate JVM, this new session is also encountering the same NoClassDefFoundError.
It seems that JVMs are sharing code loaded in memory even though each user has its own JVM, as described in the SO question what java virtual machine will do while executing multiple java applications
Does anyone know if it's possible to turn off capability of different JVMs sharing code loaded in memory when these are in the same Citrix server?
By reading this article, I know that each java application will run in a specific Java Virtual Machine Instance. So if I execute the following commands("Java -jar test1.jar","Java -jar test2.jar", I will get two processes in the system. And If each command used the default heap size, for example, 256M. The total memory cost is 512M, is that right?
Also I have other questions:
Is the Java virtual Machine a daemon process, start up with the system?
When I execute "java -jar test1.jar", it will create an instance of Java Virtual Machine, then execute the main function. Does it mean every running java application is a sub thread or process of Java Virtual Machine?
Is each running java application individual, other application can not get variable, method, constant, etc, from this running java application?
If one running java application is crashed, will it affect other running java application?
PS: I googled and got lots of different answers, I was totally confused. Anyone who can help me on this kind of questions or even more depth of Java virtual Machine. For example, How it works.
The JVM is a standard process, just like any other. As such there's no implicit communication or state sharing between the two. Each will have their own heap, threads etc. If you kill one it won't affect the other.
What will get shared are the code pages of the JVM itself. The kernel is intelligent enough to identify the same binary (any binary -not just the JVM) running twice and reuse the image. This only applies to the actual binary code - not its state. See here for more info re. Linux.
The JVM isn't a daemon process, but could be started upon system startup as a Windows service or Unix/Linux process (via /etc/init.d scripts). This is how you'd (say) run a web service written in Java when a machine is booted up.
1) No, but there a ways to launch java applications as services with wrappers (Google for "Java service").
2) Yes.
3) You can use communication between processes (v.g. HTTP). But there are no shortcuts due to all processes being run in JVM.
4) No
For the OS, JVM like an user application. Each JVM Instance is individual.
No. JVM is normal process as others.But you can run it as deamon process.
Yes. Java application run on JVM just like you application on OS.
Yes. Each JVM thread is individual, but they can communication whith other JVMs through network,RMI...
It depends. Normally they are individual, but if a JVM crash cause the OS crash, other JVMs will be effected.
I am going to create a Desktop application in Java. I don't know whether user having JAVA VIRTUAL MACHINE or not.
My questions are:
I want to know whether its possible to run Java application in a machine that doesn't contains JVM.
If its possible, I want to know whether this case is possible, I will created a application say MyApps, I will to convert this as .exe file and if user clicks MyApps.exe it should run my Java application and install JVM and run in that machine.
Use webstart, and to install java automatically, see the great comment of Andrew Thompson.
Then the user is free to use Solaris, Mac or Linux if he likes.
and updates for the JVM will be shipped to him. You don't need to rollout a new update for every bugfix in the JVM or Java-libs.
You can run a Java application on a machine which does not have a JVM, provided you install the version of Java you need first.
You can create an application which will install java as required and then run your application. However you cannot write this in Java (unless you have a JVM installed already)
This is not completely pointless as many system have some version of Java but may not have the version you need.
Nope, you need a JVM to run the Java bytecode. The only solution would be to transform the bytecode into a different executable format.
Finally I got the solution for running my Java application without Java Virtual Machine, by bundling JRE in the exe file. I did this by using the following link.
It's really working awesome.
JAVA without JVM using Launch4J
If we maintain the Directory Path as given we can get .exe with JRE bundled that will run without JVM.
I use C# to program and use ReadProcessMemory to read the memory of other processes running on the system. However, I'm unsure how to read the memory of a java applet that is running inside a browser? Has anyone tackled this before?
Thanks.
Since 6u10 the default Java PlugIn runs outside of the browser process(es). The process should be readily identifiable as a Java executable with PlugIn classes added to the bootstrap class path.
If the JVM is executing as part of the browser process, I suspect you won't be able to do this easily. The closest you'll be able to get is to measure the browser memory consumption.
However you could measure the memory consumption of the standalone applet viewer whilst running your applet, and then perhaps derive the applet memory consumption from that.