IBM 64-bit Runtime Environment for Windows AMD64/EM64T architecture, Java 2 Technology Edition, Version 1.4.2

Runtime Environment User Guide


Copyright information

Note: Before using this information and the product it supports, be sure to read the general information under Notices.

This edition of the User Guide applies to the IBM 64-bit Runtime Environment for Windows AMD64/EM64T architecture, Java 2 Technology Edition, Version 1.4.2, and to all subsequent releases, modifications, and service refreshes, until otherwise indicated in new editions.

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Preface

The IBM(R) 64-bit Runtime Environment for Windows(R) AMD64/EM64T architecture, Java(TM) 2 Technology Edition, Version 1.4.2 contains the Java(TM) Virtual Machine (JVM), core Sun Microsystems Java classes, and supporting files. The Runtime Environment does not contain all the classes that are found in the SDK package.

The User Guide provides general information about the Runtime Environment and specific information about any differences in the IBM(R) implementation of the Runtime Environment compared with the Sun implementation. Read this User Guide in conjunction with the more extensive documentation on the Sun Web site: http://java.sun.com.

The Runtime Environment is supported on Microsoft(R) Windows(R) XP 64-Bit Edition, Windows Server 2003 64-bit edition, and Windows Vista. The Windows Vista Aero interface is not supported.

The IBM JVM Diagnostics Guide provides more detailed information about the IBM JVM.

Note that the SDK includes the Runtime Environment for Windows, which enables you only to run Java applications. If you have installed the SDK, the Runtime Environment is included.

The terms "Runtime Environment" and "Java Virtual Machine" are used interchangeably throughout this User Guide.

Contents

Copyright information
Preface
Using the Runtime Environment
Obtaining the IBM build and version number
Migrating from other IBM JVMs
Launching a Java application
Summary of commands
Options
Globalization of the java command
Working with classpaths
Executing a Java file automatically
Specifying garbage collection policy
The Just-In-Time (JIT) compiler
Implementing the Connection Handler Pool for RMI
How the JVM processes signals
Signals used by the JVM
Linking a native code driver to the signal-chaining library
Working with an Input Method Editor
Using the more command with Traditional Chinese
Any comments on this User Guide?
Notices
Trademarks

Using the Runtime Environment

Obtaining the IBM build and version number

To obtain the IBM build and version number, at a command prompt type:

java -version

Migrating from other IBM JVMs

The IBM 64-bit Runtime Environment for Windows AMD64/EM64T architecture, Java 2 Technology Edition, Version 1.4.2 contains new versions of the IBM Java Virtual Machine and the Just-In-Time (JIT) compiler. If you are migrating from an IBM Runtime Environment on another platform, you might need to be aware of the following differences:

Launching a Java application

The java tool launches a Java application. It requires an initial Java class name as parameter. If you do not supply this, a usage message is printed.

The JVM searches for the initial class, and other classes that are used, in three sets of locations: the bootstrap classpath, the installed extensions, and the user classpath. Arguments after the class name or JAR file name are passed to the main function.

The javaw command is identical to java, except that javaw has no associated console window. Use javaw when you do not want a command prompt window to appear. The javaw launcher displays a dialog box with error information if a launch fails.

Summary of commands

The java and javaw command have the following syntax:

java [ options ] class [ arguments ... ]
java [ options ] -jar file.jar [ arguments ... ]
javaw [ options ] class [ arguments ... ]
javaw [ options ] -jar file.jar [ arguments ... ]

Items that are within brackets are optional.

options
Command-line options.
class
Name of the class to invoke.
file.jar
Name of the jar file to invoke. It is used only with -jar.
argument
Argument passed to the main function.

If the -jar option is specified, the named JAR file contains class and resource files for the application, with the startup class indicated by the Main-Class manifest header.

Options

The launcher has a set of standard options that are supported on the current runtime environment and will be supported in future releases. In addition, there is a set of nonstandard options.

Standard options

Nonstandard options

The -X options listed below are nonstandard and subject to change without notice.

Globalization of the java command

The java command and other java launcher commands (such as javaw) allow a class name to be specified as any character that is in the character set of the current locale.

You can also specify any Unicode character in the class name and arguments by using java escape sequences. To do this, you must specify -Xargencoding. To specify a Unicode character, use escape sequences in the form \u####, where # is a hexadecimal digit (0 through 9, A through F).

Alternatively, to specify that the class name and command arguments are in UTF8 encoding, use -Xargencoding:utf8, or in ISO8859_1 encoding use-Xargencoding:latin.

The java and javaw commands give translated output messages. These messages differ based on the locale in which Java is running. The detailed error descriptions and other debug information that is returned by java are in English.

Working with classpaths

You can specify a class name as a complete file name including a full path and the .class extension. In previous versions, you could specify only the class that was relative to the CLASSPATH, and the .class extension was not allowed. Use of the complete file name permits you to launch a java application from your desktop or file launcher. If you specify a .class file with path and extension, the specified path is put into the CLASSPATH. For example, the command java c:\myapp\thisapp.class is equivalent to specifying java -classpath c:\myapp thisapp.

Executing a Java file automatically

To set a java class or jar file to execute automatically from the file use Tools->Folder Options->File Type option of Windows Explorer. Alternatively, at a command prompt type:

assoc .class=javaclass 
ftype javaclass=C:\Program Files\IBM\Java142\jre\bin\java.exe %l %*
Notes:
  1. The %l is the letter l and not the number 1.
  2. If your Java is installed in a directory other than C:\Program Files\IBM\Java142\, substitute your directory.

Specifying garbage collection policy

The -Xgcpolicy JVM runtime option specifies garbage collection policy.

-Xgcpolicy takes the values optthruput (the default), optavgpause, or gencon. The option controls garbage collector behavior, making tradeoffs between throughput of the application and overall system and the pause times that are caused by garbage collection.

The format of the option and its values is:

-Xgcpolicy:optthruput

-Xgcpolicy:optavgpause

-Xgcpolicy:gencon

Pause time

When an application's attempt to create an object cannot be satisfied immediately from the available space in the heap, the garbage collector is responsible for identifying unreferenced objects (garbage), deleting them, and returning the heap to a state in which the immediate and subsequent allocation requests can be satisfied quickly. Such garbage collection cycles introduce occasional unexpected pauses in the execution of application code. Because applications grow in size and complexity, and heaps become correspondingly larger, this garbage collection pause time tends to grow in size and significance. The default garbage collection value, optthruput, delivers very high throughput to applications, but at the cost of these occasional pauses, which can vary from a few milliseconds to many seconds, depending on the size of the heap and the quantity of garbage.

Pause time reduction

The JVM uses two techniques to reduce pause times:

The -Xgcpolicy:optavgpause command-line option requests the use of concurrent garbage collection to reduce significantly the time that is spent in garbage collection pauses. Concurrent GC reduces the pause time by performing some garbage collection activities concurrently with normal program execution to minimize the disruption caused by the collection of the heap. The -Xgcpolicy:optavgpause option also limits the effect of increasing the heap size on the length of the garbage collection pause. The -Xgcpolicy:optavgpause option is most useful for configurations that have large heaps. With the reduced pause time, you might experience some reduction of throughput to your applications.

During concurrent garbage collection a significant amount of time is wasted identifying relatively long-lasting objects that cannot then be collected. If the GC concentrates on just those objects that are most likely to be recyclable, you can further reduce pause times for some applications. Generational GC achieves this by dividing the heap into two "generations", the "nursery" and the "tenure" areas. Objects are placed in one of these areas depending on their age. The nursery is the smaller of the two and contains younger objects; the tenure is larger and contains older objects. Objects are first allocated to the nursery; if they survive long enough they are promoted to the tenure area eventually.

Generational GC depends on most objects not lasting long. Generational GC reduces pause times by concentrating the effort to reclaim storage on the nursery because it has the most recyclable space. Rather than occasional but lengthy pause times to collect the entire heap, the nursery is collected more frequently and, if the nursery is small enough, pause times are comparatively short. However, generational GC has the drawback that, over time, the tenure area might become full if too many objects last too long. To minimize the pause time when this situation occurs, use a combination of concurrent GC and generational GC. The -Xgcpolicy:gencon option requests the combined use of concurrent and generational GC to help minimize the time that is spent in any garbage collection pause.

Environments with very full heaps

If the Java heap becomes nearly full, and very little garbage is to be reclaimed, requests for new objects might not be satisfied quickly because no space is immediately available. If the heap is operated at near-full capacity, application performance might suffer regardless of which of the above options is used; and, if requests for more heap space continue to be made, the application receives an OutofMemory exception, which results in JVM termination if the exception is not caught and handled. At this point the JVM will produce a "javadump" diagnostic file. In these conditions, you are recommended either to increase the heap size by using the -Xmx option, or to reduce the number of application objects in use.

The Just-In-Time (JIT) compiler

The Just-In-Time (JIT) compiler (j9jit22.dll) dynamically generates machine code for frequently used bytecode sequences in Java applications and applets while they are running.

The Runtime Environment for Windows includes the JIT, which is enabled by default. You can disable the JIT to help isolate a problem with a Java application, an applet, or the compiler itself.

To disable the JIT, use the -Xint option. At the command prompt window where you run the application, type:

    java -Xint class

To verify whether or not the JIT is enabled, type at a command prompt:

    java -version

If the JIT is in use, a message is displayed that includes:

(JIT enabled)

If the JIT is not in use, a message is displayed that includes:

(JIT disabled)

For more information about the JIT, see the Diagnostics Guide.

Implementing the Connection Handler Pool for RMI

Thread pooling for RMI Connection Handlers is not enabled by default.

To enable the connection pooling implemented at the RMI TCPTransport level, set the option

-Dsun.rmi.transport.tcp.connectionPool=true (or any non-null value) 

This version of the Runtime Environment does not have any setting that you can use to limit the number of threads in the connection pool.

For more information, see the Sun Java site: http://java.sun.com.

How the JVM processes signals

When a signal is raised that is of interest to the JVM, a signal handler is called. This signal handler determines whether it has been called for a Java or non-Java thread.

If the signal is for a Java thread, the JVM takes control of the signal handling. If an application handler for this signal is installed and you did not specify the -Xnosigchain command-line option, after the JVM has finished processing, the application handler for this signal is called.

If the signal is for a non-Java thread, and the application that installed the JVM had previously installed its own handler for the signal, control is given to that handler. Otherwise, if the signal is requested by the JVM or Java application, the signal is ignored or the default action is taken.

The exception to this rule is on Windows, where for a signal generated externally (for example, when you enter CTRL-BREAK) a new thread is created to execute the signal handler. In this case, the JVM signal handler performs its processing and if an application handler for this signal is installed and you did not specify the -Xnosigchain command-line option, the application handler for this signal is called.

For exception and error signals, the JVM either:

For information about writing a launcher that specifies the above hooks, see: http://www-106.ibm.com/developerworks/java/library/i-signalhandling/. This item was written for Java V1.3.1, but still applies to later versions.

For interrupt signals, the JVM also enters a controlled shutdown sequence, but this time it is treated as a normal termination that:

The shutdown is identical to the shutdown initiated by a call to the Java method System.exit().

Other signals that are used by the JVM are for internal control purposes and do not cause it to terminate. The only control signal of interest is SIGBREAK, which causes a Javadump to be generated.

Signals used by the JVM

Table 1 below shows the signals that are used by the JVM. The signals are grouped in the table by type or use, as follows:

Table 1. Signals used by the JVM
Signal Name Signal type Description Disabled by -Xrs
SIGSEGV Exception Incorrect access to memory (write to inaccessible memory) No
SIGILL Exception Illegal instruction (attempt to invoke an unknown machine instruction) No
SIGFPE Exception Floating point exception (divide by zero) No
SIGABRT Error Abnormal termination. The JVM raises this signal whenever it detects a JVM fault. No
SIGINT Interrupt Interactive attention (CTRL-C). JVM exits normally. Yes
SIGTERM Interrupt Termination request. JVM will exit normally. Yes
SIGBREAK Control A break signal from a terminal. JVM uses this for taking Javadumps. No

Use the -Xrs (reduce signal usage) option to prevent the JVM from handling most signals. For more information, see Sun's Java application launcher page at http://java.sun.com/j2se/1.4.2/docs/tooldocs/windows/java.html.

Signals 2 (SIGINT), 4 (SIGILL), 6 (SIGABRT), 8 (SIGFPE), and 11 (SIGSEGV) on JVM threads cause the JVM to shut down; therefore, an application signal handler should not attempt to recover from these unless it no longer requires the services of the JVM.

Linking a native code driver to the signal-chaining library

The Runtime Environment contains signal-chaining. Signal-chaining enables the JVM to interoperate more efficiently with native code that installs its own signal handlers.

Signal-chaining enables an application to link and load the shared library jsig.dll before msvcrt.dll. The jsig.dll library ensures that calls to signal() are intercepted so that their handlers do not replace the JVM's signal handlers. Instead, these calls save the new signal handlers, or "chain" them behind the handlers that are installed by the JVM. Later, when any of these signals are raised and found not to be targeted at the JVM, the preinstalled handlers are invoked.

To use jsig.dll, link it with the application that creates or embeds a JVM.

Working with an Input Method Editor

When working with an Input Method Editor (IME), you are advised that character composition should be completed and the candidate selected before using the workspace for any other operation.

Using the more command with Traditional Chinese

Users of Traditional Chinese should not pipe the output from Java applications directly into the more command. Instead, direct the output to a temporary file and then view the file separately.

Any comments on this User Guide?

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Notices

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Trademarks

IBM, developerWorks, iSeries, pSeries, AS/400, and PowerPC are trademarks or registered trademarks of International Business Machines Corporation in the United States, or other countries, or both.

Java and all Java-based trademarks are trademarks of Sun Microsystems, Inc. in the United States, other countries, or both.

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This product includes software developed by the Apache Software Foundation http://www.apache.org/.