What’s happening with my Java shared classes?

When your Java application ends it prints information like

JVMSHRC168I Total shared class bytes read=78435794. Total bytes stored=522418
JVMSHRC818I Total unstored bytes due to the setting of shared cache soft max is 0. Unstored AOT bytes due to the setting of -Xscmaxaot is o.
Unstored JIT bytes due to the setting of -Xscmaxjitdata is 0.

So you can see what amount of space is used.

Displaying activity as Java starts

-Xshareclasses:verboseIO,…

For example

 -Xshareclasses:verboseIO,verbose,name=zoweGW,cacheDirPerm=0777,cacheDir=/u/tmp/zowec/" 

This produces output like

Entries which were found and used

Found class java/lang/Object in shared cache for class-loader id 0. 
Found class java/lang/J9VMInternals in shared cache for class-loader id 0. 

Entries which were not found…but added.

The following shows that the entry was not found in the share classes cache, but was added successfully. Next time this would give ” found and used” above.

Failed to find class java/security/Guard in shared cache for class-loader id 0. 
Stored class java/security/Guard in shared cache for class-loader id 0 with 
URL /Z31B/usr/lpp/java/J8.0_64/J21.0_64/lib/modules (index 0). 

What classes were not found and not added?

Failed to find class org/yaml/snakeyaml/constructor/Construct in shared cache for class-loader id 0. 
Failed to find class org/yaml/snakeyaml/constructor/Construct in shared cache for class-loader id 2. 

When Java option -verbose:class was enabled, there was additional information

class load: org.yaml.snakeyaml.constructor.Construct from: 
jar:nested:/u/tmp/zowep/components/gateway/bin/gateway-service.jar/!BOOT-INF/lib/snakeyaml-2.2.jar!/ 

The jar:nested means it is a jar within a bigger jar. The shared class support does not support this.

To exploit the shared classes function it seems to be better to have individual .jar files rather than one large jar file with lots of jar files within it.

The loader id 2 is from the spring framework loaded. This loads the nested jar files.

What classes were loaded from where?

With the Java option -verbose:class (not a shared classes cache option) this reports on classes used.

 class load: java/net/URLClassLoader$SharedClassMetaDataCache from: jrt:/java.base 
 class load: java/net/URLClassLoader$SharedClassMetaData from: jrt:/java.base 

What’s in the shared cache?

-Xshareclasses:…,printStats

Use the printStats command

/usr/lpp/java/J21.0_64/bin/java -Xshareclasses:cacheDir=/u/tmp/zowec,name=zoweGW,printStats,

to display a summary of the cache

cache layer                          = 0                       
cache size                           = 83885520                
softmx bytes                         = 83885520                
free bytes                           = 31274608                
Reserved space for AOT bytes         = -1                      
Maximum space for AOT bytes          = -1                      
Reserved space for JIT data bytes    = -1                      
Maximum space for JIT data bytes     = 31457280                
Metadata bytes                       = 752058                  
Metadata % used                      = 1%                      
Class debug area size                = 6680576                 
Class debug area used bytes          = 5576890                 
Class debug area % used              = 83%                     
                                                               
ROMClass bytes                       = 35210448                
AOT bytes                            = 8456750                 
JIT data bytes                       = 1150472                 
Zip cache bytes                      = 0                       
Startup hint bytes                   = 240                     
Data bytes                           = 360368                  
                                                               
# ROMClasses                         = 14554                   
# AOT Methods                        = 3510                    
# Classpaths                         = 2                       
# URLs                               = 0                       
# Tokens                             = 0                       
# Zip caches                         = 0                       
# Startup hints                      = 2                       
# Stale classes                      = 0                       
% Stale classes                      = 0%                      
                                                               
                                                               
Cache is 62% full                                              
                                                               
Cache is accessible to current user = true                     

Class debug area % used. If this is close to 100% your cache is too small, you will need to destroy the cache, and make it bigger.

-Xshareclasses:…,printAllStats

The printAllStats command provides the information in printStats, and detailed information like

1: 0x000002003ADA0DB4 CLASSPATH 
 /Z31B/usr/lpp/java/J8.0_64/J21.0_64/lib/modules 
1: 0x000002003ADA0D80 ROMCLASS: java/lang/Object at 0x00000200364581E0. 
 Index 0 in classpath 0x000002003ADA0DB4 
 ROMMETHOD: <init> Signature: ()V Address: 0x00000200364583D8 
 ROMMETHOD: clone Signature: ()Ljava/lang/Object; Address: 0x00000200364583F4 
 ROMMETHOD: equals Signature: (Ljava/lang/Object;)Z Address: 0x000002003645841C 
 ROMMETHOD: finalize Signature: ()V Address: 0x000002003645844C 
 ROMMETHOD: getClass Signature: ()Ljava/lang/Class; Address: 0x0000020036458484 
 ROMMETHOD: hashCode Signature: ()I Address: 0x00000200364584A0 
 ROMMETHOD: notify Signature: ()V Address: 0x00000200364584C0 
 ROMMETHOD: notifyAll Signature: ()V Address: 0x00000200364584D8 
 ROMMETHOD: toString Signature: ()Ljava/lang/String; Address: 0x00000200364584F0 
 ROMMETHOD: wait Signature: ()V Address: 0x000002003645852C 
 ROMMETHOD: wait Signature: (J)V Address: 0x0000020036458554 
 ROMMETHOD: wait Signature: (JI)V 

This shows for class java/lang/Object coming from /Z31B/usr/lpp/java/J8.0_64/J21.0_64/lib/modules there were several methods.