Switching to Eclipse OpenJ9

If you are already familiar with HotSpot command-line options but want the advantages of Eclipse OpenJ9™, the following information will prove helpful. In all cases, check individual topics for minor discrepancies in the way these options might work.

Note: For information about HotSpot equivalences and differences for items other than command-line options, see New to Eclipse OpenJ9?

Compatible options

You can use the following command-line options in OpenJ9, just as you did in HotSpot; you can continue to use the HotSpot option in OpenJ9 without having to change your code:

Option	Usage
`-X`	Displays help on nonstandard options.
`-Xbootclasspath`	Specifies the search path for bootstrap classes and resources.
`-Xcheck:jni`	Runs additional checks for JNI functions during VM startup.
`-Xfuture`	Turns on strict class-file format checks.
`-Xint`	Runs an application in interpreted-only mode.
`-Xlog`	Some forms of `-Xlog` that enable garbage collection logging are recognized. (Equivalent to `-Xverbosegclog`).
`-Xmn`	Sets the initial and maximum size of the new area when using -Xgcpolicy:gencon.
`-Xms`	Sets the initial size of the heap. (Equivalent to `-XX:InitialHeapSize`)
`-Xmx`	Specifies the maximum size of the object memory allocation pool. (Equivalent to `-XX:MaxHeapSize`)
`-Xnoclassgc`	Disables class garbage collection (GC).
`-Xrs`	Prevents the OpenJ9 run time environment from handling signals.
`-Xss`	Sets the Java™ thread stack size. (Equivalent to `-XX:ThreadStackSize`). Note: Unlike HotSpot, this option applies only to the Java stack. OpenJ9 has a separate native stack for operating system threads (see `-Xmso`)
`-Xverify:mode`	Enables or disables the verifier.
`-XX:ConcGCThreads`	Configures the number of GC mutator background threads.
`-XX:[+\|-]AlwaysPreTouch`	Enables/disables committing of memory during initial heap inflation or heap expansion.
`-XX:[+\|-]CompactStrings`	Enables/disables `String` compression. Note: This option is enabled by default in OpenJ9 on Java 11 and later. In the earlier versions, this option is disabled by default.
`-XX:DiagnoseSyncOnValueBasedClasses=<number>`	Configure warnings for value-based classes.
`-XX:[+\|-]DisableExplicitGC`	Enables/disables explicit `System.gc()` calls. (Alias for `-Xdisableexplicitgc` / `-Xenableexplicitgc`)
`-XX:[+\|-]ExitOnOutOfMemoryError`	Triggers VM shutdown on out-of-memory conditions.
`-XX:[+\|-]HeapDumpOnOutOfMemory`	Enables/disables dumps on out-of-memory conditions.
`-XX:HeapDumpPath`	Specifies a directory for all VM dumps including heap dumps, javacores, and system dumps. (Alias for `-Xdump:directory`)
`-XX:[+\|-]IgnoreUnrecognizedVMOptions`	Specifies whether to ignore unrecognized top-level VM options.
`-XX:InitialHeapSize`	Sets the initial size of the heap. (Alias for `-Xms`)
`-XX:InitialRAMPercentage`	Sets the initial size of the Java heap as a percentage of total memory.
`-XX:MaxDirectMemorySize`	Sets a limit on the amount of memory that can be reserved for all direct byte buffers.
`-XX:MaxHeapSize`	Specifies the maximum size of the object memory allocation pool. (Alias for `-Xmx`)
`-XX:MaxRAMPercentage`	Sets the maximum size of the Java heap as a percentage of total memory.
`-XX:OnOutOfMemoryError`	Runs specified commands when a `java.lang.OutOfMemoryError` is thrown. (Equivalent to `-Xdump:tool:events=systhrow,filter=java/lang/OutOfMemoryError,exec=`)
`-XX:ParallelCMSThreads`	Configures the number of GC mutator background threads.
`-XX:ParallelGCThreads`	Configures the number of GC threads.
`-XX:[+\|-]PrintCodeCache`	Prints code cache usage when the application exits.
`-XX:[+\|-]ShowHiddenFrames`	Specifies whether generated hidden `MethodHandle` frames are displayed in a stack trace. Note: Unlike HotSpot, this option doesn't require the `+UnlockDiagnosticVMOptions` option.
`-XX:[+\|-]UseCompressedOops`	Disables compressed references in 64-bit JVMs. (See also `-Xcompressedrefs`)
`-XX:[+\|-]UseContainerSupport`	Sets a larger fraction of memory to the Java heap when the VM detects that it is running in a container.

Equivalent options

These HotSpot command-line options have equivalents in OpenJ9 that are not specified in the same way, but perform a related function:

HotSpot Option	OpenJ9 Option	Usage
`-Xcomp`	`-Xjit:count=0`¹	`-Xcomp` disables interpreted method invocations.
`-Xgc`	`-Xgcpolicy`²	Configuring your garbage collection policy.
`-XX:+UseNUMA`	`-Xnuma:none`³	Controls non-uniform memory architecture (NUMA) awareness.

Notes:

HotSpot uses -Xcomp to force compilation of methods on first invocation. However, this option is deprecated. Whilst it can be used for compatibility, using -Xjit:count=0 is preferred.
HotSpot uses -Xgc to both select policies and configure them; OpenJ9 uses -Xgcpolicy to select policies, reserving -Xgc for configuration.
In HotSpot, NUMA awareness is turned off by default and is turned on by using the -XX:+UseNUMA option. Conversely, the OpenJ9 VM automatically enables NUMA awareness and uses -Xnuma:none to turn it off.
- If you were previously using HotSpot in its default mode, you must now explicitly turn off NUMA awareness in OpenJ9.
- If you are used to using -XX:+UseNUMA in HotSpot, you no longer need to explicitly turn on NUMA awareness; it's on by default.

Creating compatible behavior

You can set the following options to make OpenJ9 behave in the same way as HotSpot.

Option	Usage
`-Djava.lang.string.substring.nocopy=true`	Avoid String sharing by `String.substring()`.
`-Xnuma:none`	Disable non-uniform memory architecture (NUMA) awareness.
`-XX:+CompactStrings`	Enables `String` compression.
`-XX:[+\|-]HandleSIGABRT`	Force handling of SIGABRT signals to be compatible with HotSpot.

Compatible environment variables

The JAVA_TOOL_OPTIONS environment variable can be used to set command line options as described in OpenJ9 command-line options and Environment variables.