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From Ralph Goers <ralph.go...@dslextreme.com>
Subject Re: Enums and Custom Levels
Date Sun, 26 Jan 2014 05:23:07 GMT
Yeah, I missed the {} as I modified the class before I tried to compile it.

You can’t just use an AtomicInteger because you might be adding a duplicate to the map,
in which case you have incorrectly incremented the ordinal.  I used simple synchronization.

Unfortunately, because the extended levels are no longer enums that implement an interface,
the code I used to instantiate them during configuration doesn’t work. Class.getEnumConstants
was nice because it doesn’t through a SecurityException and most of the other methods I
could use do. So now I am looking for another way.  Everything else works though.

Ralph

On Jan 25, 2014, at 9:13 PM, Remko Popma <remko.popma@gmail.com> wrote:

> Ralph,
> I copied Nick's code _as is_ and had no compile errors.
> The class is abstract, but instances are defined in the static block as:
> OFF = new Level("OFF", 0) {}; // note the {}: this creates an anonymous concrete subclass
> 
> I agree that read access needs to be synchronized as well, not just write access (the
constructor).
> I experimented with several options: 
> * synchronizing on plain Object in both constructor and when accessing the static Map(s)
> * a ReentrantReadWriteLock
> * a lock-free implementation
> 
> I decided against ReentrantReadWriteLock as it has more overhead than plain synchronized
access and the write access (in the constructors) is going to be extremely rare: not worth
paying the overhead in the more common reads. It is also cumbersome to code.
> 
> The lock-free implementation uses an AtomicInteger for the ordinals, and an AtomicReference
for the Map<String, Level>.
> In the constructor, create a new Map<String, Level> instance based on the old copy,
add the new instance, and try to call compareAndSet to replace the old instance with the new
instance. Retry on failure.
> 
> Finally, simply synchronizing on the constructorLock object in the Level.toLevel() and
Level.values() methods may be simplest. 
> 
> Which of the last two is best depends on how often the toLevel() and values() levels
are called.
> It turns out they are only called during reconfiguration, so no real need to optimize
these methods.
> I would argue that simple synchronization may be best in this case.
> 
> Remko
>  
> 
> 
> On Sun, Jan 26, 2014 at 1:49 PM, Ralph Goers <ralph.goers@dslextreme.com> wrote:
> As I am working on this I just want to point out a number of issues with the code below:
> 
> 1. The class is abstract. The static block is doing a bunch of new Level() invocations
which obviously generate compile errors on an abstract class.  I had to make it be a non-abstract
class.
> 2. As I pointed out before there is no way to access the “standard” levels as an
enum. I have addressed that.
> 3. Although the constructor is synchronized access to the Map is not. Trying to get from
the map while a Level is being added will result in a ConcurrentModificationException. I am
using a ConcurrentMap instead.
> 3. The constructor requires synchronization because it is modifying both the map and
the ordinal. However, since this isn’t an enum the ordinal value is of dubious value. Removing
that would allow the removal of the synchronization in the constructor. I am considering that
but I haven’t done it yet.
> 4. Your example of creating the extension shows doing a new Level(). This doesn’t work
because a) the class is abstract and b) the constructor is protected. I am leaving the constructor
protected so extension will require doing new ExtendedLevel(name, value) and creating a constructor.
Not requiring that means applications can do a new Level() anywhere and I am opposed to allowing
that.
> 
> Ralph
> 
> On Jan 23, 2014, at 12:42 AM, Nick Williams <nicholas@nicholaswilliams.net> wrote:
> 
> > Okay, I finally got a minute to read all of these emails, and...
> >
> > EVERYBODY FREEZE!
> >
> > What if I could get you an extensible enum that required no interface changes and
no binary-incompatible changes at all? Sound too good to be true? I proposed this months ago
(LOG4J2-41) and it got shot down multiple times, but as of now I've heard THREE people say
"extensible enum" in this thread, so here it is, an extensible enum:
> >
> > public abstract class Level implements Comparable<Level>, Serializable {
> >    public static final Level OFF;
> >    public static final Level FATAL;
> >    public static final Level ERROR;
> >    public static final Level WARN;
> >    public static final Level INFO;
> >    public static final Level DEBUG;
> >    public static final Level TRACE;
> >    public static final Level ALL;
> >
> >
> >    private static final long serialVersionUID = 0L;
> >    private static final Hashtable<String, Level> map;
> >    private static final TreeMap<Integer, Level> values;
> >    private static final Object constructorLock;
> >
> >
> >    static {
> >        // static variables must be constructed in certain order
> >        constructorLock = new Object();
> >        map = new Hashtable<String, Level>();
> >        values = new TreeMap<Integer, Level>();
> >        OFF = new Level("OFF", 0) {};
> >        FATAL = new Level("FATAL", 100) {};
> >        ERROR = new Level("ERROR", 200) {};
> >        WARN = new Level("WARN", 300) {};
> >        INFO = new Level("INFO", 400) {};
> >        DEBUG = new Level("DEBUG", 500) {};
> >        TRACE = new Level("TRACE", 600) {};
> >        ALL = new Level("ALL", Integer.MAX_VALUE) {};
> >    }
> >
> >
> >    private static int ordinals;
> >
> >
> >    private final String name;
> >    private final int intLevel;
> >    private final int ordinal;
> >
> >
> >    protected Level(String name, int intLevel) {
> >        if(name == null || name.length() == 0)
> >            throw new IllegalArgumentException("Illegal null Level constant");
> >        if(intLevel < 0)
> >            throw new IllegalArgumentException("Illegal Level int less than zero.");
> >        synchronized (Level.constructorLock) {
> >            if(Level.map.containsKey(name.toUpperCase()))
> >                throw new IllegalArgumentException("Duplicate Level constant [" +
name + "].");
> >            if(Level.values.containsKey(intLevel))
> >                throw new IllegalArgumentException("Duplicate Level int [" + intLevel
+ "].");
> >            this.name = name;
> >            this.intLevel = intLevel;
> >            this.ordinal = Level.ordinals++;
> >            Level.map.put(name.toUpperCase(), this);
> >            Level.values.put(intLevel, this);
> >        }
> >    }
> >
> >
> >    public int intLevel() {
> >        return this.intLevel;
> >    }
> >
> >
> >    public boolean isAtLeastAsSpecificAs(final Level level) {
> >        return this.intLevel <= level.intLevel;
> >    }
> >
> >
> >    public boolean isAtLeastAsSpecificAs(final int level) {
> >        return this.intLevel <= level;
> >    }
> >
> >
> >    public boolean lessOrEqual(final Level level) {
> >        return this.intLevel <= level.intLevel;
> >    }
> >
> >
> >    public boolean lessOrEqual(final int level) {
> >        return this.intLevel <= level;
> >    }
> >
> >
> >    @Override
> >    @SuppressWarnings("CloneDoesntCallSuperClone")
> >    public Level clone() throws CloneNotSupportedException {
> >        throw new CloneNotSupportedException();
> >    }
> >
> >
> >    @Override
> >    public int compareTo(Level other) {
> >        return intLevel < other.intLevel ? -1 : (intLevel > other.intLevel
? 1 : 0);
> >    }
> >
> >
> >    @Override
> >    public boolean equals(Object other) {
> >        return other instanceof Level && other == this;
> >    }
> >
> >
> >    public Class<Level> getDeclaringClass() {
> >        return Level.class;
> >    }
> >
> >
> >    @Override
> >    public int hashCode() {
> >        return this.name.hashCode();
> >    }
> >
> >
> >    public String name() {
> >        return this.name;
> >    }
> >
> >
> >    public int ordinal() {
> >        return this.ordinal;
> >    }
> >
> >
> >    @Override
> >    public String toString() {
> >        return this.name;
> >    }
> >
> >
> >    public static Level toLevel(String name) {
> >        return Level.toLevel(name, Level.DEBUG);
> >    }
> >
> >
> >    public static Level toLevel(String name, Level defaultLevel) {
> >        if(name == null)
> >            return defaultLevel;
> >        name = name.toUpperCase();
> >        if(Level.map.containsKey(name))
> >            return Level.map.get(name);
> >        return defaultLevel;
> >    }
> >
> >
> >    public static Level[] values() {
> >        return Level.values.values().toArray(new Level[Level.values.size()]);
> >    }
> >
> >
> >    public static Level valueOf(String name) {
> >        if(name == null)
> >            throw new IllegalArgumentException("Unknown level constant [" + name
+ "].");
> >        name = name.toUpperCase();
> >        if(Level.map.containsKey(name))
> >            return Level.map.get(name);
> >        throw new IllegalArgumentException("Unknown level constant [" + name + "].");
> >    }
> >
> >
> >    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
> >        return Enum.valueOf(enumType, name);
> >    }
> >
> >
> >    // for deserialization
> >    protected final Object readResolve() throws ObjectStreamException {
> >        return Level.valueOf(this.name);
> >    }
> > }
> >
> > Extending it is easy:
> >
> > public final class ExtendedLevels {
> >    public static final Level MY_LEVEL = new Level("MY_LEVEL", 250) {};
> > }
> >
> > I still and have ALWAYS believed this was the best option. If we used this option,
I would be fine with not adding any new Levels because I could add them myself.
> >
> > Nick
> >
> > On Jan 22, 2014, at 7:04 PM, Remko Popma wrote:
> >
> >> This is only a problem for webapps, right?
> >> Putting log4j jars in WEB-INF/lib avoids that problem (different class loader).
> >> Apps that really want to share log4j jars with other apps would need to play
nice. Such apps would do well to use a naming convention like Gary suggests.
> >> Otherwise, the last to register would overwrite any previous level with the
same name. (Should probably emit a StatusLogger warning.)
> >>
> >> Same intLevel for different names should not be a problem.
> >>
> >>
> >> On Thursday, January 23, 2014, Gary Gregory <garydgregory@gmail.com> wrote:
> >> Playing devils advocate:
> >>
> >> What happens when different apps register levels with the same name and different
intLevels?
> >> What happens when different apps register levels with the same intLevel and
different names?
> >> Should there be a convention that custom level names be FQNs?
> >>
> >> Gary
> >>
> >>
> >> On Wed, Jan 22, 2014 at 10:05 PM, Paul Benedict <pbenedict@apache.org>
wrote:
> >> As Gary wanted, a new thread....
> >>
> >> First, each enum needs an inherit strength. This would be part of the interface.
Forgive me if the word "strength" is wrong; but it's the 100, 200, 300, etc. number that triggers
the log level. So make sure the interface contains the intLevel() method.
> >>
> >> Second, we need to know the name, right? The name probably requires a new method
since it can't be extracted from the enum anymore.
> >>
> >> public interface Level {
> >> int intLevel();
> >> String name();
> >> }
> >>
> >> PS: The intStrength() name seems hackish. What about strength() or treshold()?
> >>
> >> Third, the registration can be done manually by providing a static method (as
your did Remko) that the client needs to invoke, or you could have a class-path scanning mechanism.
For the latter, you could introduce a new annotation to be placed on the enum class.
> >>
> >> @CustomLevels
> >> public enum MyCustomEnums {
> >> }
> >>
> >> Paul
> >>
> >> On Wed, Jan 22, 2014 at 8:52 PM, Remko Popma <remko.popma@gmail.com> wrote:
> >> Paul, can you give a bit more detail?
> >>
> >> I tried this: copy the current Level enum to a new enum called "Levels" in the
same package (other name would be fine too). Then change Level to an interface (removing the
constants and static methods, keeping only the non-static methods). Finally make the Levels
enum implement the Level interface.
> >>
> >> After this, we need to do a find+replace for the references to Level.CONSTANT
to Levels.CONSTANT and Level.staticMethod() to Levels.staticMethod().
> >>
> >> Finally, the interesting part: how do users add or register their custom levels
and how do we enable the Levels.staticLookupMethod(String, Level) to recognize these custom
levels?
> >>
> >>
> >>
> >> On Thursday, January 23, 2014, Paul Benedict <pbenedict@apache.org> wrote:
> >> Agreed. This is not an engineering per se, but really more about if the feature
set makes sense.
> >>
> >> Well if you guys ever look into the interface idea, you'll give log4j the feature
of getting enums to represent custom levels. That's pretty cool, IMO. I don't know if any
other logging framework has that and that would probably get some positive attention. It shouldn't
be so hard to do a find+replace on the code that accepts Level and replace it with another
name. Yes, there will be some minor refactoring that goes with it, but hard? It shouldn't
be.
> >>
> >> A name I propose for the interface is LevelDefinition.
> >>
> >> Paul
> >>
> >>
> >> On Wed, Jan 22, 2014 at 6:48 PM, Gary Gregory <garydgregory@gmail.com>
wrote:
> >> Hi, I do not see this as an engineering problem but more a feature set definition
issue. So while there may be lots of more or less internally complicated ways of solving this
with interfaces, makers and whatnots, the built in levels are the most user friendly.
> >>
> >> I have have lots of buttons, knobs and settings on my sound system that I do
not use, just like I do not use all the methods in all the classes in the JRE...
> >>
> >> Gary
> >>
> >>
> >>
> >>
> >> --
> >> E-Mail: garydgregory@gmail.com | ggregory@apache.org
> >> Java Persistence with Hibernate, Second Edition
> >> JUnit in Action, Second Edition
> >> Spring Batch in Action
> >> Blog: http://garygregory.wordpress.com
> >> Home: http://garygregory.com/
> >> Tweet! http://twitter.com/GaryGregory
> >
> >
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