Thanks Ioi - looks good.
thanks,
Karen
On Oct 30, 2014, at 1:26 AM, Ioi Lam wrote:
> OK, here's the latest version. I removed the synchronization but kept the
> resolveClass just for completeness, even if it currently does nothing.
>
> class Launcher$AppClassLoader {
> ....
> public Class<?> loadClass(String name, boolean resolve)
> throws ClassNotFoundException
> {
> int i = name.lastIndexOf('.');
> if (i != -1) {
> SecurityManager sm = System.getSecurityManager();
> if (sm != null) {
> sm.checkPackageAccess(name.substring(0, i));
> }
> }
>
> if (ucp.knownToNotExist(name)) {
> // The class of the given name is not found in the parent
> // class loader as well as its local URLClassPath.
> // Check if this class has already been defined dynamically;
> // if so, return the loaded class; otherwise, skip the parent
> // delegation and findClass.
> Class<?> c = findLoadedClass(name);
> if (c != null) {
> if (resolve) {
> resolveClass(c);
> }
> return c;
> }
> throw new ClassNotFoundException(name);
> }
>
> return (super.loadClass(name, resolve));
> }
>
> Thanks
>
> - Ioi
>
> On 10/29/14, 12:10 PM, Mandy Chung wrote:
>>
>> On 10/29/2014 7:16 AM, Karen Kinnear wrote:
>>> Sorry, I was confused about who wrote what.
>>>
>>> Sounds like David and I are in agreement that you can remove the
>>> synchronization - I believe that would be much cleaner.
>>
>> I agree that the class loader lock is not really needed in
>> the knownToNotExist case as it's checking if the class is
>> loaded or not. Good catch, Karen.
>>
>> Mandy
>>
>>> And resolveClass does nothing and is final so no worries there.
>>>
>>> thanks,
>>> Karen
>>>
>>> On Oct 29, 2014, at 2:37 AM, David Holmes wrote:
>>>
>>>> On 29/10/2014 4:04 PM, Ioi Lam wrote:
>>>>> On 10/28/14, 7:34 PM, David Holmes wrote:
>>>>>> Hi Karen,
>>>>>>
>>>>>> I haven't been tracking the details of this and am unclear on the
>>>>>> overall caching strategy however ...
>>>>>>
>>>>>> On 29/10/2014 8:49 AM, Karen Kinnear wrote:
>>>>>>> Ioi,
>>>>>>>
>>>>>>> Looks good! Thanks to all who have contributed!
>>>>>>>
>>>>>>> A couple of minor comments/questions:
>>>>>>>
>>>>>>> 1. jvm.h (hotspot and jdk)
>>>>>>> All three APIs talk about loader_type, but the code uses loader.
>>>>>>>
>>>>>>> 2. Launcher.java
>>>>>>> To the best of my understanding - the call to findLoadedClass does
>>>>>>> not require synchronizing on the class loader lock,
>>>>>>> that is needed to ensure find/define atomicity - so that we do not
>>>>>>> call defineClass twice on the same class - i.e. in
>>>>>>> loadClass - it is needed around the findLoadedClass /
>>>>>>> findClass(defineClass) calls. This call is just a SystemDictionary
>>>>>>> lookup
>>>>>>> and does not require the lock to be held.
>>>>>> If the class can be defined dynamically - which it appears it can
>>>>>> (though I'm not sure what that means) - then you can have a race
>>>>>> between the thread doing the defining and the thread doing the
>>>>>> findLoadedClass. By doing findLoadedClass with the lock held you
>>>>>> enforce some serialization of the actions, but there is still a race.
>>>>>> So the only way the lock could matter is if user code could trigger
>>>>>> the second thread's lookup of the class after the lock has been taken
>>>>>> by the thread doing the dynamic definition - whether that is possible
>>>>>> depends on what this dynamic definition actually is.
>>>>>>
>>>>> I copied the code from ClassLoader.loadClass, which does it it a
>>>>> synchronized block:
>>>>>
>>>>> class ClassLoader {
>>>>> protected Class<?> loadClass(String name, boolean resolve)
>>>>> throws ClassNotFoundException
>>>>> {
>>>>> synchronized (getClassLoadingLock(name)) {
>>>>> // First, check if the class has already been loaded
>>>>> Class<?> c = findLoadedClass(name);
>>>>> if (c == null) {
>>>>> long t0 = System.nanoTime();
>>>>> try {
>>>>> if (parent != null) {
>>>>> c = parent.loadClass(name, false);
>>>>> } else {
>>>>> c = findBootstrapClassOrNull(name);
>>>>> }
>>>>> } catch (ClassNotFoundException e) {
>>>>> // ClassNotFoundException thrown if class not found
>>>>> // from the non-null parent class loader
>>>>> }
>>>>>
>>>>> if (c == null) {
>>>>> // If still not found, then invoke findClass in order
>>>>> // to find the class.
>>>>> long t1 = System.nanoTime();
>>>>> c = findClass(name);
>>>>>
>>>>> // this is the defining class loader; record the stats
>>>>> sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
>>>>> sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
>>>>> sun.misc.PerfCounter.getFindClasses().increment();
>>>>> }
>>>>> }
>>>>> if (resolve) {
>>>>> resolveClass(c);
>>>>> }
>>>>> return c;
>>>>> }
>>>>> }
>>>>>
>>>>> So I guess it should look like this in Launcher$AppClassLoader, just to
>>>>> ensure the same things are done (regardless of whether it's necessary or
>>>>> not)?
>>>> In ClassLoader.loadClass it is providing atomicity across a number of
>>>> actions in the worst-case:
>>>> - checking for already loaded; if not then
>>>> - try to load through parent; if not then
>>>> - findClass (which will do defineClass)
>>>>
>>>> You don't have those atomicity constraints because you are only doing one
>>>> thing - checking to see if the class is loaded.
>>>>
>>>> Your locking is probably harmless but those are famous last words when it
>>>> comes to classloading. :)
>>>>
>>>>> Does resolveClass need to be done inside the synchronized block?
>>>> Depends on whether it depends on the classloader locking to prevent
>>>> concurrent resolve attempts.
>>>>
>>>> David
>>>> -----
>>>>
>>>>> class Launcher$AppClassLoader {
>>>>> public Class<?> loadClass(String name, boolean resolve)
>>>>> throws ClassNotFoundException
>>>>> {
>>>>> int i = name.lastIndexOf('.');
>>>>> if (i != -1) {
>>>>> SecurityManager sm = System.getSecurityManager();
>>>>> if (sm != null) {
>>>>> sm.checkPackageAccess(name.substring(0, i));
>>>>> }
>>>>> }
>>>>>
>>>>> if (ucp.knownToNotExist(name)) {
>>>>> // The class of the given name is not found in the parent
>>>>> // class loader as well as its local URLClassPath.
>>>>> // Check if this class has already been defined
>>>>> dynamically;
>>>>> // if so, return the loaded class; otherwise, skip the
>>>>> parent
>>>>> // delegation and findClass.
>>>>>> >from here
>>>>> * synchronized (getClassLoadingLock(name)) {**
>>>>> ** Class<?> c = findLoadedClass(name);**
>>>>> ** if (c != null) {**
>>>>> ** if (resolve) {**
>>>>> ** resolveClass(c);**
>>>>> ** }**
>>>>> ** return c;**
>>>>> ** }**
>>>>> ** }*
>>>>> <<to here
>>>>> throw new ClassNotFoundException(name);
>>>>> }
>>>>>
>>>>> return (super.loadClass(name, resolve));
>>>>> }
>>>>>
>>>>> Thanks
>>>>> - Ioi
>>>>>
>>>>>
>>>>>> David
>>>>>>
>>>>>>> David H and Mandy - does that make sense to you both?
>>>>>>>
>>>>>>> thanks,
>>>>>>> Karen
>>>>>>>
>>>>>>> On Oct 28, 2014, at 12:38 AM, Ioi Lam wrote:
>>>>>>>
>>>>>>>> On 10/27/14, 7:04 PM, Mandy Chung wrote:
>>>>>>>>> On 10/27/2014 3:32 PM, Ioi Lam wrote:
>>>>>>>>>> Hi David, I have update the latest webrev at:
>>>>>>>>>>
>>>>>>>>>> http://cr.openjdk.java.net/~iklam/8061651-lookup-index-open-v3/
>>>>>>>>>>
>>>>>>>>> The update looks good. Thanks.
>>>>>>>>>
>>>>>>>>>> This version also contains the JDK test case that Mandy requested:
>>>>>>>>>>
>>>>>>>>>> http://cr.openjdk.java.net/~iklam/8061651-lookup-index-open-v3/jdk/test/sun/misc/URLClassPath/EnableLookupCache.java.html
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>> What I request to add is a test setting the system property
>>>>>>>>> (-Dsun.cds.enableSharedLookupCache=true) and continue to load class
>>>>>>>>> A and B. Removing line 44-58 should do it and also no need to set
>>>>>>>>> -Dfoo.foo.bar.
>>>>>>>>>
>>>>>>>> Do you mean change the test to call
>>>>>>>> System.setProperty("sun.cds.enableSharedLookupCache", "true")?
>>>>>>>>
>>>>>>>> But we know that the property is checked only once, before any app
>>>>>>>> classes are loaded. So calling System.setProperty in an application
>>>>>>>> class won't test anything.
>>>>>>>>> It'd be good if you run this test and turn on the debug traces to
>>>>>>>>> make sure that the application class loader and ext class loader
>>>>>>>>> will start up with the lookup cache enabled and make up call to the
>>>>>>>>> VM. As it doesn't have the app cds archive, it will invalidate the
>>>>>>>>> cache right away and continue the class lookup with null cache array.
>>>>>>>> In the latest code, if CDS is not available, lookupCacheEnabled will
>>>>>>>> be set to false inside the static initializer of URLClassPath:
>>>>>>>>
>>>>>>>> private static volatile boolean lookupCacheEnabled
>>>>>>>> =
>>>>>>>> "true".equals(VM.getSavedProperty("sun.cds.enableSharedLookupCache"));
>>>>>>>>
>>>>>>>> later, when the boot/ext/app loaders call into here:
>>>>>>>>
>>>>>>>> synchronized void initLookupCache(ClassLoader loader) {
>>>>>>>> if ((lookupCacheURLs = getLookupCacheURLs(loader)) != null) {
>>>>>>>> lookupCacheLoader = loader;
>>>>>>>> } else {
>>>>>>>> // This JVM instance does not support lookup cache.
>>>>>>>> disableAllLookupCaches();
>>>>>>>> }
>>>>>>>> }
>>>>>>>>
>>>>>>>> their lookupCacheURLs[] fields will all be set to null. As a result,
>>>>>>>> getLookupCacheForClassLoader and knownToNotExist0 will never be called.
>>>>>>>>
>>>>>>>> I can add a DEBUG_LOOKUP_CACHE trace inside disableAllLookupCaches
>>>>>>>> to print "lookup cache disabled", and check for that in the test. Is
>>>>>>>> this OK?
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>> - Ioi
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>
>
