Why JDK's JConsole seek virtual machines using both jvmstat and attach api? - jvm

Question

I'm writing some JVM instance related application and looking at open source to see how it's solve some problems. The JConsole from JDK7 collects running VMs in two ways (look at source licensed by GPL2 in jdk/src/share/classes/sun/tools/jconsole/LocalVirtualMachine.java). The first is jvmstat way, code like this:
private static void getMonitoredVMs(Map<Integer, LocalVirtualMachine> map) {
MonitoredHost host;
Set vms;
try {
host = MonitoredHost.getMonitoredHost(new HostIdentifier((String)null));
vms = host.activeVms();
} catch (java.net.URISyntaxException sx) {
throw new InternalError(sx.getMessage());
} catch (MonitorException mx) {
throw new InternalError(mx.getMessage());
}
for (Object vmid: vms) {
if (vmid instanceof Integer) {
int pid = ((Integer) vmid).intValue();
String name = vmid.toString(); // default to pid if name not available
boolean attachable = false;
String address = null;
try {
MonitoredVm mvm = host.getMonitoredVm(new VmIdentifier(name));
// use the command line as the display name
name = MonitoredVmUtil.commandLine(mvm);
attachable = MonitoredVmUtil.isAttachable(mvm);
address = ConnectorAddressLink.importFrom(pid);
mvm.detach();
} catch (Exception x) {
// ignore
}
map.put((Integer) vmid,
new LocalVirtualMachine(pid, name, attachable, address));
}
}
}
Second is attach way, and looks like this:
private static void getAttachableVMs(Map<Integer, LocalVirtualMachine> map) {
List<VirtualMachineDescriptor> vms = VirtualMachine.list();
for (VirtualMachineDescriptor vmd : vms) {
try {
Integer vmid = Integer.valueOf(vmd.id());
if (!map.containsKey(vmid)) {
boolean attachable = false;
String address = null;
try {
VirtualMachine vm = VirtualMachine.attach(vmd);
attachable = true;
Properties agentProps = vm.getAgentProperties();
address = (String) agentProps.get(LOCAL_CONNECTOR_ADDRESS_PROP);
vm.detach();
} catch (AttachNotSupportedException x) {
// not attachable
} catch (IOException x) {
// ignore
}
map.put(vmid, new LocalVirtualMachine(vmid.intValue(),
vmd.displayName(),
attachable,
address));
}
} catch (NumberFormatException e) {
// do not support vmid different than pid
}
}
}
My question: why it uses two different tools for retrieving virtual machines list? I know that through attach api you can list VMs running by this same JRE only, but jvmstat can give you list of all VMs running with any JRE versions. I have tested JRE/JDK 7 32 and 64-bit only because theys and newer are my target, sadly on windows only. Is not suffiecient to use jvmstat only? Is there any case when some VM is visible by attach api, but jvmstat can't see it?

Answer 1

Usually, for finding all jvm process, attach api is equal to jvmstat. But in some customized circumstances, it's diffrent. And the diffrent is com.sun.tools.attach.spi.AttachProvider.
e.g. in Windows platform, jvmstat finds java process by listing all files in the directory %TEMP%/hsperfdata_caoxudong(In linux, it is /tmp/hsperfdata_caoxudong). And attach api finds java processes by AttachProvider instance. jdk provides a default an AttachProvider implementation, which depends on your OS platform. In Windows Platform, the implementation is sun.tools.attach.WindowsAttachProvider. In its listVirtualMachines method, if isTempPathSecure method return false, it will iterate all processes, and find all processes, which loaded library "jvm.dll". You can install your own AttachProvider implementation to find java processes with your own ways, and the result may be diffrent with jvmstat.
The installation of AttachProvider is here.