java.exe : Java Glossary

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java.exe
java.exe is the JVM (Java Virtual Machine) runtime or JIT (Just In Time) that lets you execute your class files. It comes bundled with the JDK. If you install Java 1.8.0_25 in the default directory you should find it in J:\Program Files\java\jdk1.8.0_25\ \bin\java.exe. The java.exe that comes in the stripped down JRE (Java Runtime Environment) install for end users that does not include the compiler etc. is used to be called jre.exe. In  Java version 1.3 or later, there is only one java.exe. jre.exe is gone. For all practical purposes you can treat jre.exe as identical to java.exe.
Long Command Lines Multiple java.exes
CLASSPATH And File Naming Recipes Aborting
The Java.exe Command Line Under the Hood
_JAVA_OPTIONS Default File and Console Encoding
Parameters with Embedded Blanks javac.exe
Awkward Characters in Command Line Parameters Learning More
Jars Links
The key thing to understand about java.exe is that it wants the name of a class on the command line, not the name of a class file. It could have been made smart enough to eat either, but it wasn’t.

The other thing to understand is that java.exe and javac.exe were written by two different teams of people who never had coffee together. That’s why the command line switches are so infuriatingly inconsistent.

All applications that you run with java.exe (such as HelloWorld.java ) must

public static void main ( String[] args )
method.

Long Command Lines

javac.exe has the @file.txt method to build up long command lines out of pieces. Unfortunately you can’t do that with java.exe. If your command line is too long what can you do?

CLASSPATH And File Naming Recipes

Here are my simplified rules for using CLASSPATH and naming the files on the java.exe command line:
  1. Use Java version 1.1 or later. Configure your SET SET CLASSPATH= to clear it out. Avoid JDK (Java Development Kit) 1.0 if you can because its CLASSPATH is more complicated since it has to help java find the standard classes.
  2. In all that follows, everything is strictly case-sensitive.
  3. To run a HelloWorld.class app, in the default package in C:\MyDir\ , use
    CD \MyDir
    java.exe -ea -classpath . HelloWorld
    Note that the -classpath must come before the HelloWorld classname. To remember the order, you can think of it like this. java.exe needs to know the classpath before it can search for the class. If you get them reversed you will just get a mysterious NoClassDefFoundError.
  4. The following will not work to run a HelloWorld.class app, in the default package
  5. To run a HelloWorld.class app in J:\com\mindprod\mypackage\ , in package com.mindprod.mypackage, use
    CD \
    java.exe -ea -classpath . com.mindprod.mypackage.HelloWorld
  6. The following will not work to run a HelloWorld.class app in J:\com\mindprod\mypackage\ , in package com.mindprod.mypackage:
    CD E:\com\mindprod\mypackage
    java.exe -ea -classpath . com.mindprod.mypackage.HelloWorld
    For that to work, you must use smartj instead of java.exe.
  7. The following will not work to run a HelloWorld.class app in

  8. The following will work to run a HelloWorld.class app in

    REM For this to work, you must specify the class to execute,
    REM com.mindprod.mypackage.HelloWorld in the manifest Main-Class entry.
    CD \AnyDir
    java.exe -ea -jar HelloWorld.jar
  9. If for any reason the examples shown do not work with your version of java.exe, try various combinations of com.mindprod.mypackage.HelloWorld, com/mindprod/mypackage/HelloWorld and com\mindprod\mypackage\HelloWorld.

The Java.exe Command Line

Java.exe command line switches
Option effect
-client This is the default. Use the version of the JVM optimised for client apps
-cp .;C:\java\classes.zip… list directories in which to look for classes. It is sometimes spelled out longhand -classpath. Infuriatingly, javac.exe won’t let you use the -cp shortcut. Starting with Java version 1.2 you don’t add classes.zip, (or rt.jar) to the classpath.
-d32 use 32 bit JVM if possible.
-d64 use 64 bit JVM if possible.
-debug enable remote JAVA debugging. For ordinary debugging, you probably want -verbose or -g instead.
-ea Turn on assertion checking.
-g include debug code to create stack traces with variable names and line numbers.
-javaagent:myagent.jar define a jar containing a java.lang.instrument agent to use for monitoring code. Java version 1.5 or later. Note the quirky extra colon.
-help print out info on options. Trust what it says over what I say here. Knowledge keeps no better than fish.
-jar C:\java\myjar.jar… Names jar to look inside for class files. This jar must have a manifest Main-Class which gives the name of the class to execute. You may not specify it on the command line.
-noclassgc disable class garbage collection.
-noverify do not verify any class.
-prof:java.prof output profiling data to .\java.prof.
-server use the version of the JVM optimised for server apps. This only works with the private JRE java.exe that comes with the JDK in J:\Program Files\java\jdk1.8.0_25\ /bin/java.exe, not the public JRE in C:\Program Files\java\jre1.8.0_25\ /bin/java.exe.
-showOptions This option does not yet exist. I have requested it with
Oracle RFE number JI-9013765 : java.exe -showOptions
. It displays the values of all the options, showing the resulting merge of specified options, defaults and computer defaults.
-verbose turn on verbose mode.
-version print out the build version. Can also be used to specify the version you want.
-verbose:class display the name of each class as it is loaded.
-verbosegc print a message when garbage collection occurs.
-verify verify all classes when read in.
-verifyremote verify classes read in over the network.
-Xfuture Recommended to test all class files for strict conformance with coming mandatory standards.
-Xms2000m set the initial Java heap size, in megabytes. g/gigabytes are not supported. The default is computed based on the system configuration. You can burn this, and similar java.exe switches in at compile time with the javac.exe -J switch. You can also put them into a SET parameter and
set jopts=-Xms4m
...
java.exe %jopts% MyClass
-Xincgc Use incremental garbage collection. You have shorter pauses, but it takes 10% more overhead.
-Xint Use a pure interpreter, not the Hotspot JIT .
-Xmx5000m set the maximum Java heap size, in megabytes. g/gigabytes are not supported. The default is computed based on the system configuration.
When tweaking the memory configuration options, try a variety of constellations of options to discover which actually works better. Bigger is not necessarily better. What is best depends on your hardware and what else in running in your machine at the time. It would be nice if there were some utility that could automatically tweak these parameters depending on the current conditions and past history.
-Xoss300k set the maximum Java stack size for any thread, in 1024 byte chunks.
-Xss64k set the maximum native stack size for any thread, in 1024 byte chunks.
-XX:+DoEscapeAnalysis The option directs HotSpot to look for objects that are created and referenced by a single thread within the scope of a method compilation. Allocation is omitted for such non-escaping objects, and their fields are treated as local variables, often residing in machine registers. Synchronization on non-escaping objects is also elided.
-XX:+PrintOptoAssembly Display the generated assembly code, not the byte code, the CPU-specific assembly code generated by HotSpot.
-XX:+UseCompressedOops The option can improve performance of the 64-bit JRE when the Java object heap is less than 32 gigabytes in size. In this case, HotSpot compresses object references to 32 bits, reducing the amount of data that it must process.
-XX:+UseParNewGC Use concurrent garbage collection to avoid pauses.
-XX:MaxPermSize=64m Size of the permanent generation memory pool in megabytes for long lived objects.
-XX:xxxx There are many extended esoteric options
There is no command line option to set the locale or language or even the default encoding. You can however set system properties with the -D option.

_JAVA_OPTIONS

If you find yourself using the same options over and over on your java.exe command line, you set up an environment variable to contain your default options. e.g. In Windows:
set _JAVA_OPTIONS=-Xms64m -Xmx128m -Dawt.useSystemAAFontSettings=lcd
In Linux:
export _JAVA_OPTIONS='-Xms64m -Xmx128m -Dawt.useSystemAAFontSettings=lcd'

You still have to manually insert that SET parm on the java.exe command line. e.g.

java.exe %_JAVA_OPTIONS -jar some.jar

_JAVA_OPTIONS is just an arbitrary name. It has no special meaning to java.exe.

Here are the settings that IntelliJ suggests for 64-bit java.exe. If you have plenty of RAM (Random Access Memory) you might experiment bumping up some of the memory values.

Parameters with Embedded Blanks

In Unix, java.exe is just a script. You can modify the way it passes parameters from the command line into the JVM by changing this line:
exec $JAVA_HOME/bin/$ARCH/$THREADS_TYPE/$PROG $@
Unfortunately every OS (Operating System) ’s command processor has its own way of dealing with passing parameters that contain spaces. To make matters worse, java.exe then reparses them to pass to your main method. With two cooks, much can go wrong. The easiest solution is just to avoid blanks or quotes or other suspicious characters in your parameters. Encode them as _ or some other safe character and convert them back yourself.

On windows you can enclose your blank-containing parameters in "s and they will be stripped off before your main program sees them.

Awkward Characters in Command Line Parameters

I have tested the following only in Windows 2000. You may have to experiment with your own OS to see how much of this applies.

Jars

java.exe is ever so much easier to understand and use when all the class files are inside a single jar. In that case the member names are identical to the package names. You can hide the console by using javaw.exe (java without) instead of using java.exe.

Multiple java.exes

If you do a search/filefind you will discover more copies of java.exe than you can shake a stick at. There are java.exes for different JRE/JDK versions you make have installed, and there are at least two java.exes installed for each JDK version. Which one gets used? It depends on your path. The one first on the path is the one that gets used, usually the one in C:\Windows\system32\ or C:\WINNT\system32\ . There in one in the JRE at C:\Program Files\java\jre1.8.0_25\ \bin and a second in the JDK at J:\Program Files\java\jdk1.8.0_25\ \bin. If you have more than on JDK installed you will have yet another extra pair for each version. Sometime an application that uses Java will install its own private copy of java.exe as well.

To complicate things further the java.exe in system32 is just a dummy. It looks in the registry and then decides which real java.exe to use. The last JVM installed gets to be the one used, even if it is older. To switch JVM s, you must normally reinstall the one you want.

I have found that sometimes, if you just type plain java.exe you will get the JDK version and sometimes the JRE version. I think Oracle may have switched horses at some point. It used to be the JRE , but now it seems to be the JDK. You can look at the system property sun.boot.library.path= C:\Program Files\java\jre1.8.0_25\ \bin or sun.boot.library.path= J:\Program Files\java\jdk1.8.0_25\ \bin to determine which one is being used. For explicit control, type "C:\Program Files\java\jre1.8.0_25\ \bin\java.exe" or " J:\Program Files\java\jdk1.8.0_25\ \bin\java.exe"

Java.exe Variants
Java.exe location Examines
registry?
Supports
-server?
path C:\Windows\System32\java.exe
JRE C:\Program Files\java\jre1.8.0_25\ \bin\java.exe
JDK J:\Program Files\java\jdk1.8.0_25\ \bin\java.exe
The path java.exe is usually first of the path, or you type just plain java.exe it will start. It then looks in the registry to find the official JRE, usually the one on C:.

If you explicitly invoke the JRE java.exe, it just executes, without checking the official JRE, since it notices it is living in a JRE directory.

If you explicitly invoke the JDK java.exe, it just executes, without checking the official JRE. Further, it has additional support for the -server option that the JRE java.exe lacks.

How do you select which java.exe to run?

Here are some possible ways:

Aborting

You can abort java.exe by hitting ctrl-C. You can abort and get a dump of what was going on at the time with the threads with Ctrl+Shift+Break. For anything fancier, you need a debugger.

Under the Hood

If you are curious how java.exe works, you can examine the c source code for launcher/java.c in src.zip. You can see how it finds the correct JVM to load.

Learning More

Oracle’s JDK Tool Guide to Java.exe : available:


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