Java HotSpot VM Options

VM options categorized in two types:

1. Standard options
2. Non-standard options

For Standard Options please refer the follwoing links depending on your plateform

Windows: http://java.sun.com/j2se/1.5.0/docs/tooldocs/windows/java.html

Solaris: http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/java.html

Linux: http://java.sun.com/j2se/1.5.0/docs/tooldocs/linux/java.html

Non-standard options are further categorized in two:

• Options that begin with -X are non-standard (not guaranteed to be supported on all VM implementations), and are subject to change without notice in subsequent releases of the JDK.
  
• Options that are specified with -XX are not stable and are not recommended for casual use. These options are subject to change without notice.

Note: For -X options, please refer the links mentioned above.

Some Useful -XX Options

• Boolean options are turned on with -XX:+ and turned off with -XX:-.
• Numeric options are set with -XX:=. Numbers can include 'm' or 'M' for megabytes, 'k' or 'K' for kilobytes, and 'g' or 'G' for gigabytes (for example, 32k is the same as 32768).
• String options are set with -XX:=, are usually used to specify a file, a path, or a list of commands

Flags marked as manageable are dynamically writeable through the JDK management interface (com.sun.management.HotSpotDiagnosticMXBean API) and also through JConsole.

Behavioral Options

Option and Default Value	Description
-XX:-AllowUserSignalHandlers	Do not complain if the application installs signal handlers. (Relevant to Solaris and Linux only.)
-XX:AltStackSize=16384	Alternate signal stack size (in Kbytes). (Relevant to Solaris only, removed from 5.0.)
-XX:-DisableExplicitGC	Disable calls to System.gc(), JVM still performs garbage collection when necessary.
-XX:+FailOverToOldVerifier	Fail over to old verifier when the new type checker fails. (Introduced in 6.)
-XX:+HandlePromotionFailure	The youngest generation collection does not require a guarantee of full promotion of all live objects. (Introduced in 1.4.2 update 11) [5.0 and earlier: false.]
-XX:+MaxFDLimit	Bump the number of file descriptors to max. (Relevant to Solaris only.)
-XX:PreBlockSpin=10	Spin count variable for use with -XX:+UseSpinning. Controls the maximum spin iterations allowed before entering operating system thread synchronization code. (Introduced in 1.4.2.)
-XX:-RelaxAccessControlCheck	Relax the access control checks in the verifier. (Introduced in 6.)
-XX:+ScavengeBeforeFullGC	Do young generation GC prior to a full GC. (Introduced in 1.4.1.)
-XX:+UseAltSigs	Use alternate signals instead of SIGUSR1 and SIGUSR2 for VM internal signals. (Introduced in 1.3.1 update 9, 1.4.1. Relevant to Solaris only.)
-XX:+UseBoundThreads	Bind user level threads to kernel threads. (Relevant to Solaris only.)
-XX:-UseConcMarkSweepGC	Use concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1)
-XX:+UseGCOverheadLimit	Use a policy that limits the proportion of the VM's time that is spent in GC before an OutOfMemory error is thrown. (Introduced in 6.)
-XX:+UseLWPSynchronization	Use LWP-based instead of thread based synchronization. (Introduced in 1.4.0. Relevant to Solaris only.)
-XX:-UseParallelGC	Use parallel garbage collection for scavenges. (Introduced in 1.4.1)
-XX:-UseParallelOldGC	Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)
-XX:-UseSerialGC	Use serial garbage collection. (Introduced in 5.0.)
-XX:-UseSpinning	Enable naive spinning on Java monitor before entering operating system thread synchronizaton code. (Relevant to 1.4.2 and 5.0 only.) [1.4.2, multi-processor Windows platforms: true]
-XX:+UseTLAB	Use thread-local object allocation (Introduced in 1.4.0, known as UseTLE prior to that.) [1.4.2 and earlier, x86 or with -client: false]
-XX:+UseSplitVerifier	Use the new type checker with StackMapTable attributes. (Introduced in 5.0.)[5.0: false]
-XX:+UseThreadPriorities	Use native thread priorities.
-XX:+UseVMInterruptibleIO	Thread interrupt before or with EINTR for I/O operations results in OS_INTRPT. (Introduced in 6. Relevant to Solaris only.)

Performance Options

Option and Default Value	Description
-XX:+AggressiveOpts	Turn on point performance compiler optimizations that are expected to be default in upcoming releases. (Introduced in 5.0 update 6.)
-XX:CompileThreshold=10000	Number of method invocations/branches before compiling [-client: 1,500]
-XX:LargePageSizeInBytes=4m	Sets the large page size used for the Java heap. (Introduced in 1.4.0 update 1.) [amd64: 2m.]
-XX:MaxHeapFreeRatio=70	Maximum percentage of heap free after GC to avoid shrinking.
-XX:MaxNewSize=size	Maximum size of new generation (in bytes). Since 1.4, MaxNewSize is computed as a function of NewRatio. [1.3.1 Sparc: 32m; 1.3.1 x86: 2.5m.]
-XX:MaxPermSize=64m	Size of the Permanent Generation. [5.0 and newer: 64 bit VMs are scaled 30% larger; 1.4 amd64: 96m; 1.3.1 -client: 32m.]
-XX:MinHeapFreeRatio=40	Minimum percentage of heap free after GC to avoid expansion.
-XX:NewRatio=2	Ratio of new/old generation sizes. [Sparc -client: 8; x86 -server: 8; x86 -client: 12.]-client: 4 (1.3) 8 (1.3.1+), x86: 12]
-XX:NewSize=2.125m	Default size of new generation (in bytes) [5.0 and newer: 64 bit VMs are scaled 30% larger; x86: 1m; x86, 5.0 and older: 640k]
-XX:ReservedCodeCacheSize=32m	Reserved code cache size (in bytes) - maximum code cache size. [Solaris 64-bit, amd64, and -server x86: 48m; in 1.5.0_06 and earlier, Solaris 64-bit and and64: 1024m.]
-XX:SurvivorRatio=8	Ratio of eden/survivor space size [Solaris amd64: 6; Sparc in 1.3.1: 25; other Solaris platforms in 5.0 and earlier: 32]
-XX:TargetSurvivorRatio=50	Desired percentage of survivor space used after scavenge.
-XX:ThreadStackSize=512	Thread Stack Size (in Kbytes). (0 means use default stack size) [Sparc: 512; Solaris x86: 320 (was 256 prior in 5.0 and earlier); Sparc 64 bit: 1024; Linux amd64: 1024 (was 0 in 5.0 and earlier); all others 0.]
-XX:+UseBiasedLocking	Enable biased locking.
-XX:+UseFastAccessorMethods	Use optimized versions of GetField.
-XX:-UseISM	Use Intimate Shared Memory. [Not accepted for non-Solaris platforms.]
-XX:+UseLargePages	Use large page memory. (Introduced in 5.0 update 5.)
-XX:+UseMPSS	Use Multiple Page Size Support w/4mb pages for the heap. Do not use with ISM as this replaces the need for ISM. (Introduced in 1.4.0 update 1, Relevant to Solaris 9 and newer.) [1.4.1 and earlier: false]
-XX:+StringCache	Enables caching of commonly allocated strings.
-XX:AllocatePrefetchLines=1	Number of cache lines to load after the last object allocation using prefetch instructions generated in JIT compiled code. Default values are 1 if the last allocated object was an instance and 3 if it was an array.
-XX:AllocatePrefetchStyle=1	Generated code style for prefetch instructions. 0 - no prefetch instructions are generate*d*, 1 - execute prefetch instructions after each allocation, 2 - use TLAB allocation watermark pointer to gate when prefetch instructions are executed.

Debugging Options

Option and Default Value	Description
-XX:-CITime	Prints time spent in JIT Compiler. (Introduced in 1.4.0.)
-XX:ErrorFile=./hs_err_pid.log	If an error occurs, save the error data to this file. (Introduced in 6.)
-XX:-ExtendedDTraceProbes	Enable performance-impacting dtrace probes. (Introduced in 6. Relevant to Solaris only.)
-XX:HeapDumpPath=./java_pid.hprof	Path to directory or filename for heap dump. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
-XX:-HeapDumpOnOutOfMemoryError	Dump heap to file when java.lang.OutOfMemoryError is thrown. Manageable. (Introduced in 1.4.2 update 12, 5.0 update 7.)
-XX:OnError=";"	Run user-defined commands on fatal error. (Introduced in 1.4.2 update 9.)
-XX:OnOutOfMemoryError="; "	Run user-defined commands when an OutOfMemoryError is first thrown. (Introduced in 1.4.2 update 12, 6)
-XX:-PrintClassHistogram	Print a histogram of class instances on Ctrl-Break. Manageable. (Introduced in 1.4.2.) The jmap -histo command provides equivalent functionality.
-XX:-PrintConcurrentLocks	Print java.util.concurrent locks in Ctrl-Break thread dump. Manageable. (Introduced in 6.) The jstack -l command provides equivalent functionality.
-XX:-PrintCommandLineFlags	Print flags that appeared on the command line. (Introduced in 5.0.)
-XX:-PrintCompilation	Print message when a method is compiled.
-XX:-PrintGC	Print messages at garbage collection. Manageable.
-XX:-PrintGCDetails	Print more details at garbage collection. Manageable. (Introduced in 1.4.0.)
-XX:-PrintGCTimeStamps	Print timestamps at garbage collection. Manageable (Introduced in 1.4.0.)
-XX:-PrintTenuringDistribution	Print tenuring age information.
-XX:-TraceClassLoading	Trace loading of classes.
-XX:-TraceClassLoadingPreorder	Trace all classes loaded in order referenced (not loaded). (Introduced in 1.4.2.)
-XX:-TraceClassResolution	Trace constant pool resolutions. (Introduced in 1.4.2.)
-XX:-TraceClassUnloading	Trace unloading of classes.
-XX:-TraceLoaderConstraints	Trace recording of loader constraints. (Introduced in 6.)
-XX:+PerfSaveDataToFile	Saves jvmstat binary data on exit.

How to Use Log4j with WebLogic Logging Services

WebLogic logging services use an implementation based on the Java Logging APIs, by default. Using the LogMBean.isLog4jLoggingEnabled attribute, you can direct the logging services to use Log4j instead.

In the Administration Console, you can specify Log4j or keep the default Java Logging implementation.

Alternatively, you can configure Log4j logging through the LogMBean interface and by adding WebLogic-specific Log4j classes, WL_HOME/server/lib/wllog4j.jar, and the log4j.jar file to the server CLASSPATH. The recommended way to do this is to place the wllog4j.jar and log4j.jar files in the DOMAIN_NAME/lib directory; there, they will get added to the server CLASSPATH dynamically during server startup.

Note: WebLogic Server does not provide a Log4j version in its distribution.

When Log4j is enabled, you get a reference to the org.apache.log4j.Logger that the server is using from the weblogic.logging.log4j.Log4jLoggingHelper class.

With a Log4j Logger reference, you can attach you own custom appender to receive the server log events; for example, you might attach an appender that sends the server log events to Syslog or the Windows Event Viewer. Additionally, you can use the Logger reference to issue log requests to WebLogic logging services; this requires that the Log4j libraries be available to your deployed application.

If your application has no requirement to interact with WebLogic logging services, package the Log4j libraries in the application's LIB directory. The server logging will continue to use the default Java Logging implementation.

Enabling Log4j Logging

To specify logging to a Log4j Logger instead of the default Java Logging:

* When you start the Administration Server, include the following Java option in the weblogic.Server command:

-Dweblogic.log.Log4jLoggingEnabled=true

* After the Administration Server has started, use the Administration Console to specify the Log4j logging implementation. See "Specifying the Logging Implementation" in the Administration Console Online Help.
* Use the WLST to set the value of the Log4jLoggingEnabled property and re-start the server.


The WLST commands to enable logging to a Log4j Logger in the Administration Server:

#invoke WLST
C:\>java weblogic.WLST

#connect WLST to an Administration Server
wls:/offline> connect('username','password')

#navigate to the writable MBean configuration tree
wls:/mydomain/serverConfig> edit()
wls:/mydomain/edit> startEdit()

#set cmo to the server log config
wls:/mydomain/edit !> cd("Servers/myserver/Log/myserver")

#set log4j logging to true
wls:/mydomain/edit/Servers/myserver/Log/myserver !> cmo.setLog4jLoggingEnabled(true)

#save and activate the changes
wls:/mydomain/edit/Servers/myserver/Log/myserver !> save()
wls:/mydomain/edit/Servers/myserver/Log/myserver !> activate()


You can enable Log4j for the server Logger as well as the domain Logger, which resides only on the Administration Server. The domain Log4j Logger reference is provided by invoking the weblogic.logging.log4j.Log4jLoggingHelper.getLog4jDomainLogger() method.

Log4j Code Example

import org.apache.log4j.Logger;
import weblogic.logging.log4j.Log4jLoggingHelper;
import weblogic.logging.LoggerNotAvailableException;

/**
* This example shows how to use the Log4j server Logger.
*/

public class MyLog4jTest {
public void testWLSLog4j() {
try {
Logger logger = Log4jLoggingHelper.getLog4jServerLogger();
logger.addAppender(myAppender); // The Appender is configured using either the log4j props file or other custom mechanism.
logger.info("Test log message");
} catch(LoggerNotAvailableException lex) {
System.err.println("Unable to get a reference to the log4j Logger: "+
lex.getMessage())
}
}
}



You have programmatic access to the Log4j Logger and its appenders (handlers) and layouts (formatters) for configuration purposes.

Java Logging is the default for client and server-side logging; Log4j is available only for server-side and not client-side logging.

The Concept of Classloaders

Classloaders, as the name suggests, are responsible for loading classes within the JVM. Before your class can be executed or accessed, it must become available via a classloader. Given a class name, a classloader locates the class and loads it into the JVM. Classloaders are Java classes themselves. This brings the question: if classloaders are Java classes themselves, who or what loads them?

When you execute a Java program (i.e., by typing java at a command prompt), it executes and launches a native Java launcher. By native, I mean native to your current platform and environment. This native Java launcher contains a classloader called the bootstrap classloader. This bootstrap classloader is native to your environment and is not written in Java. The main function of the bootstrap classloader is to load the core Java classes.


Figure 1; Classloader delegation hierarchy

The JVM implements two other classloaders by default. The bootstrap classloader loads the extension and application classloaders into memory. Both are written in Java. As mentioned before, the bootstrap classloader loads the core Java classes (for example, classes from the java.util package). The extension classloader loads classes that extend the core Java classes (e.g., classes from the javax packages, or the classes under the ext directory of your runtime). The application classloader loads the classes that make up your application.

All three default classloaders follow the delegation model. Before a child classloader tries to locate a class, it delegates that task to a parent. When your application requests a particular class, the application classloader delegates this request to the extension classloader, which in turn delegates it to the bootstrap classloader. If the class that you requested is a Java core class, the bootstrap classloader will make the class available to you. However, if it cannot find the class, the request returns to the extension classloader, and from there to the application classloader itself. The idea is that each classloader first looks up to its parent for a particular class. Only if the parent does not have the class does the child classloader try to look it up itself.

In application servers, each separately-deployed web application and EJB gets its own classloader (normally; this is certainly the case in WebLogic). This classloader is derived from the application classloader and is responsible for that particular EJB or web application. (Note that if an application is deployed as an EAR file--a combination of EJB and webapps--it gets one classloader, no more). This new classloader loads all classes that the webapp or EJB require that are not already part the Java core classes or the extension packages. It is also responsible for loading and unloading of classes, a feature missing from the default classloaders. This feature helps in hot deploy of applications.

When WebLogic starts up, it uses the Java-supplied application classloader to load the classes that make up its runtime. It then launches individual classloaders, derived from the Java application classloader, which load the classes for individual applications. The individual classloaders are invisible to the classloaders of the other applications; hence, classes loaded for one particular application will not be seen by another application.


Figure 2: Individual application classloaders

What if you want to make a single class available to all applications? Load it in a top-level classloader. This could be in the classpath of WebLogic. When WebLogic starts, it will automatically load this class in memory using the Java-supplied application classloader. All sub-application classloaders get access to it. However, the negatives of this approach are clear too. First, you lose the capability of hot deploy for this particular class in all individual applications. Second, any change in this class means that the server needs to be restarted, as there is no mechanism for a Java application classloader to reload classes. You will need to weigh in the pros and cons before you take this approach.