Probably you should take a look at [Chapter 6: Layouts][1]...


  [1]: http://logback.qos.ch/manual/layouts.html#conversionWord

I can&#39;t figure out why m1 is apparently memoized while m2 is not in the following:

    m1      = ((filter odd [1..]) !!)
    
    m2 n    = ((filter odd [1..]) !! n)

m1 10000000 takes about 1.5 seconds on the first call, and a fraction of that on subsequent calls (presumably it caches the list), whereas m2 10000000 always takes the same amount of time (rebuilding the list with each call). Any idea what&#39;s going on? Are there any rules of thumb as to if and when GHC will memoize a function? Thanks.

When is memoization automatic in GHC Haskell?

Although I have read the documentation on Html.HiddenFor, I&#39;ve not grasped what is it used for...

Could somebody explain its uses and give a short example?

Where should those helpers go in the code? 

What does Html.HiddenFor do?

I&#39;ve gone through all the [documentation][1] of [logback][2] and I can&#39;t find anywhere the documentation to configure the encoder&#39;s pattern when logging, such as:

&lt;!-- language: xml --&gt;

    &lt;encoder&gt;
      &lt;pattern&gt;%d{HH:mm:ss.SSS} %-4relative %-5level %logger{35} - %msg%n&lt;/pattern&gt;
    &lt;/encoder&gt;

I would like a table (like the one that log4j has) explaining the different options to configure the pattern. 

Where is the documentation of the pattern? Maybe they are defined in another project?


  [1]: http://logback.qos.ch/documentation.html
  [2]: http://logback.qos.ch/

Where is logback encoder pattern documentation

I've gone through all the <a href="http://logback.qos.ch/documentation.html" target="_blank" rel="noopener noreferrer">documentation</a> of <a href="http://logback.qos.ch/" target="_blank" rel="noopener noreferrer">logback</a> and I can't find anywhere the documentation to configure the encoder's pattern when logging, such as:

<pre><code class="hljs language-perl">&#x3C;encoder>
 &#x3C;pattern>%d{HH:mm:ss.SSS} %-4relative %-5level %logger{35} - %msg%n&#x3C;/pattern>
&#x3C;/encoder>
</code></pre>
I would like a table (like the one that log4j has) explaining the different options to configure the pattern.
Where is the documentation of the pattern? Maybe they are defined in another project?

Why does this throw `NullPointerException`

    public static void main(String[] args) throws Exception {
        Boolean b = true ? returnsNull() : false; // NPE on this line.
        System.out.println(b);
    }

    public static Boolean returnsNull() {
        return null;
    }
    
while this doesn&#39;t

    public static void main(String[] args) throws Exception {
        Boolean b = true ? null : false;
        System.out.println(b); // null
    }

?

The solution is by the way to replace `false` by `Boolean.FALSE` to avoid `null` being unboxed to `boolean` --which isn&#39;t possible. But that isn&#39;t the question. The question is *why*? Are there any references in JLS which confirms this behaviour, especially of the 2nd case?

Booleans, conditional operators and autoboxing

I&#39;m designing a new app based on JPA/Hibernate, Spring and Wicket. The distinction between the DAO and Service layers isn&#39;t that clear to me though. According to Wikipedia, DAO is 

&gt; an object that provides an abstract
&gt; interface to some type of database or
&gt; persistence mechanism, providing some
&gt; specific operations without exposing
&gt; details of the database.

I was wondering whether a DAO could contain methods that don&#39;t really have to do much with data access, but are way easier executed using a query? For example &quot;get a list of all airlines that operate on a certain set of airports&quot;? It sounds to me to be more of a service-layer method, but I&#39;m not sure if using JPA EntityManager in the service layer is an example of good practice?

DAO and Service layers (JPA/Hibernate + Spring)

Can anyone shed some light on why `Double.MIN_VALUE` is not actually the minimum value that Doubles can take? It is a positive value, and a Double can of course be negative.

I understand why it&#39;s a useful number, but it seems a very unintuitive name, especially when compared to `Integer.MIN_VALUE`. Calling it `Double.SMALLEST_POSITIVE` or `MIN_INCREMENT` or similar would have clearer semantics.

Also, what is the minimum value that Doubles can take? Is it `-Double.MAX_VALUE`? The docs don&#39;t seem to say.

Why is Double.MIN_VALUE in not negative

In class we&#39;re now learning how to build up a Spring application, even though spring isn&#39;t directly involved, we learned how to make the interfaces for DAO and service layer objects. 

Please correct me if I&#39;m wrong:
DAO layer is pretty abstract: it just contains the CRUD operations and is further used to read data.(ie: get all objects, get specific objects, etc)

Service layer: contains services to create things, and delete things, this is where business logic should be. 

Now all of this makes sense in the service layer; except &quot;updating&quot; objects. Do you just put a &quot;update&quot; function that just saves the object in your database? Or do you need to define the logic there as well? This is where my confusion is as, my understanding is objects in Spring are just POJO&#39;s. Now then who validates the data? 

Let&#39;s say I have an Object &quot;child&quot;
it has:`Name`, `SurName`, `Gender`, `Photo`, `Birthdate`
fields.
how would I name the services? Or would you just let the controller take care of validation, which doesn&#39;t seem right to me. On the other hand it wouldn&#39;t seem right either to delegate every setter that needs to be called to the service layer.

So just basically: help me with how to define saving your objects via the service layer.

Service layer and controller: who takes care of what?

How can I get the name of a Java Enum type given its value?

I have the following code which works for a **particular** Enum type, can I make it more **generic**?

	public enum Category {

        APPLE(&quot;3&quot;), 
        ORANGE(&quot;1&quot;), 

		private final String identifier;

        private Category(String identifier) {
		    this.identifier = identifier;
		}

		public String toString() {
			return identifier;
		}
		 
		public static String getEnumNameForValue(Object value){
            Category[] values = Category.values();
		    String enumValue = null;
		    for(Category eachValue : values) {
		        enumValue = eachValue.toString();

		        if (enumValue.equalsIgnoreCase(value)) {
		            return eachValue.name();
                }
		    }
            return enumValue;
        }
	}



Java getting the Enum name given the Enum Value

I can&#39;t get my head around Python&#39;s `logging` module. My needs are very simple: I just want to log everything to syslog. After reading documentation I came up with this simple test script:

    import logging
    import logging.handlers
    
    my_logger = logging.getLogger(&#39;MyLogger&#39;)
    my_logger.setLevel(logging.DEBUG)
    
    handler = logging.handlers.SysLogHandler()
    
    my_logger.addHandler(handler)
    
    my_logger.debug(&#39;this is debug&#39;)
    my_logger.critical(&#39;this is critical&#39;)

But this script does not produce any log records in syslog. What&#39;s wrong?

How to configure logging to syslog in Python?

What are the best practices for logging in Android apps in production mode, keeping in mind the following requirements : 

 1. Ability to upload logs for debugging
    
 2. Ability to clean up logs or set rollovers

Thanks

Android Production Logging Best Practice

What are the best or most useful configurations for logging with NLog? (These can be simple or complex, as long as they&#39;re useful.) 

I&#39;m thinking of examples like automatically rolling over log files at a certain size, changing the layout (log message) whether or not there is an exception, escalating the log level once an error has occurred, etc.

Here are some links:

* [NLog Demo][2]
* [Examples in the source][3]

  [2]: https://github.com/jkowalski/NLog/blob/master/examples/NLogDemo/NLog.config
  [3]: http://github.com/jkowalski/NLog/tree/master/examples/targets/Configuration%20File

Most useful NLog configurations

I would like to set level logging to DEBUG in tomcat but in console nevertheless only INFO and WARN output.
Could anybody tell me what&#39;s wrong?

My C:\tomcat\logging.properties:

    # Licensed to the Apache Software Foundation (ASF) under one or more
    # contributor license agreements.  See the NOTICE file distributed with
    # this work for additional DEBUGrmation regarding copyright ownership.
    # The ASF licenses this file to You under the Apache License, Version 2.0
    # (the &quot;License&quot;); you may not use this file except in compliance with
    # the License.  You may obtain a copy of the License at
    #
    #     http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an &quot;AS IS&quot; BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    
    handlers = 1catalina.org.apache.juli.FileHandler, 2localhost.org.apache.juli.FileHandler, 3manager.org.apache.juli.FileHandler, 4host-manager.org.apache.juli.FileHandler, java.util.logging.ConsoleHandler
    
    .handlers = 1catalina.org.apache.juli.FileHandler, java.util.logging.ConsoleHandler
    
    ############################################################
    # Handler specific properties.
    # Describes specific configuration DEBUG for Handlers.
    ############################################################
    
    1catalina.org.apache.juli.FileHandler.level = DEBUG
    1catalina.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    1catalina.org.apache.juli.FileHandler.prefix = catalina.
    
    2localhost.org.apache.juli.FileHandler.level = DEBUG
    2localhost.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    2localhost.org.apache.juli.FileHandler.prefix = localhost.
    
    3manager.org.apache.juli.FileHandler.level = DEBUG
    3manager.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    3manager.org.apache.juli.FileHandler.prefix = manager.
    
    4host-manager.org.apache.juli.FileHandler.level = DEBUG
    4host-manager.org.apache.juli.FileHandler.directory = ${catalina.base}/logs
    4host-manager.org.apache.juli.FileHandler.prefix = host-manager.
    
    java.util.logging.ConsoleHandler.level = DEBUG
    java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter
    
    
    ############################################################
    # Facility specific properties.
    # Provides extra control for each logger.
    ############################################################
    
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].level = DEBUG
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].handlers = 2localhost.org.apache.juli.FileHandler
    
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].level = DEBUG
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].handlers = 3manager.org.apache.juli.FileHandler
    
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/host-manager].level = DEBUG
    org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/host-manager].handlers = 4host-manager.org.apache.juli.FileHandler
    
    # For example, set the com.xyz.foo logger to only log SEVERE
    # messages:
    #org.apache.catalina.startup.ContextConfig.level = DEBUG
    #org.apache.catalina.startup.HostConfig.level = DEBUG
    #org.apache.catalina.session.ManagerBase.level = DEBUG
    #org.apache.catalina.core.AprLifecycleListener.level=DEBUG

Example of my log:

    INFO: Deploying configuration descriptor manager.xml
    08.11.2010 1:06:42 org.apache.catalina.startup.HostConfig deployWAR
    INFO: Deploying web application archive spring-mvc-trial.war
    08.11.2010 1:06:46 org.apache.catalina.startup.HostConfig deployDirectory
    INFO: Deploying web application directory docs
    08.11.2010 1:06:46 org.apache.catalina.startup.HostConfig deployDirectory
    INFO: Deploying web application directory examples
    08.11.2010 1:06:46 org.apache.catalina.startup.HostConfig deployDirectory
    INFO: Deploying web application directory ROOT
    08.11.2010 1:06:46 org.apache.coyote.http11.Http11AprProtocol start
    INFO: Starting Coyote HTTP/1.1 on http-8080
    08.11.2010 1:06:46 org.apache.coyote.ajp.AjpAprProtocol start
    INFO: Starting Coyote AJP/1.3 on ajp-8009
    08.11.2010 1:06:46 org.apache.catalina.startup.Catalina start
    INFO: Server startup in 3777 ms
    08.11.2010 1:09:36 org.apache.coyote.http11.Http11AprProtocol pause
    INFO: Pausing Coyote HTTP/1.1 on http-8080
    08.11.2010 1:09:36 org.apache.coyote.ajp.AjpAprProtocol pause
    INFO: Pausing Coyote AJP/1.3 on ajp-8009
    08.11.2010 1:09:37 org.apache.catalina.core.StandardService stop
    INFO: Stopping service Catalina
    08.11.2010 1:09:37 org.apache.catalina.loader.WebappClassLoader clearReferencesJdbc
    SEVERE: The web application [/spring-mvc-trial] registered the JBDC driver [com.mysql.jdbc.Driver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
    08.11.2010 1:09:37 org.apache.catalina.loader.WebappClassLoader clearReferencesThreads
    SEVERE: The web application [/spring-mvc-trial] appears to have started a thread named [MySQL Statement Cancellation Timer] but has failed to stop it. This is very likely to create a memory leak.
    08.11.2010 1:09:38 org.apache.coyote.http11.Http11AprProtocol destroy
    INFO: Stopping Coyote HTTP/1.1 on http-8080
    08.11.2010 1:09:38 org.apache.coyote.ajp.AjpAprProtocol destroy
    INFO: Stopping Coyote AJP/1.3 on ajp-8009



How to set level logging to DEBUG in Tomcat?

I&#39;m using standard python logging module in my python application:
&lt;pre&gt;
import logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(&quot;log&quot;)
while True:
  logger.debug(&#39;Stupid log message &quot; + &#39; &#39;.join([str(i) for i in range(20)]) )
  # Do something
&lt;/pre&gt;

The issue is that although debug level is not enable, that stupid log message is evaluated on each loop iteration, which harms performance badly.

__Is there any solution for this?__

In C++ we have `log4cxx` package that provides macros like this:&lt;br&gt;
`    LOG4CXX_DEBUG(logger, messasage)` &lt;br&gt;
That effectively evaluates to 
&lt;pre&gt;
if (log4cxx::debugEnabled(logger)) {
    log4cxx.log(logger,log4cxx::LOG4CXX_DEBUG, message)
}
&lt;/pre&gt;
But since there are no macros in Python (AFAIK), if there a efficient way to do logging?


Lazy logger message string evaluation

The &quot;Hello World&quot; example from [SLF4J][1] is not working for me. I guess this is because I added slf4j-log4 to my classpath. Should I configure log4j directly for the hello world to work?

    log4j:WARN No appenders could be found for logger (HelloWorld).
    log4j:WARN Please initialize the log4j system properly.
    log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.

**Update**: I added log4j initialization, and it still doesn&#39;t work:

    public static void main(String[] params) {
            org.apache.log4j.Logger.getRootLogger().addAppender(new ConsoleAppender());
            Logger logger = org.slf4j.LoggerFactory.getLogger(TestBase.class);
            logger.info(&quot;Hello World&quot;);
        }

And I&#39;m getting:

    log4j:ERROR No output stream or file set for the appender named [null].

  [1]: http://www.slf4j.org/manual.html


How to get SLF4J &quot;Hello World&quot; working with log4j?

Say you catch an exception and get the following on the standard output (like, say, the console) if you do a *e.printStackTrace()* :

    java.io.FileNotFoundException: so.txt
            at java.io.FileInputStream.&lt;init&gt;(FileInputStream.java)
            at ExTest.readMyFile(ExTest.java:19)
            at ExTest.main(ExTest.java:7)

Now I want to send this instead to a logger like, say, log4j to get the following:

    31947 [AWT-EventQueue-0] ERROR Java.io.FileNotFoundException: so.txt
    32204 [AWT-EventQueue-0] ERROR    at java.io.FileInputStream.&lt;init&gt;(FileInputStream.java)
    32235 [AWT-EventQueue-0] ERROR    at ExTest.readMyFile(ExTest.java:19)
    32370 [AWT-EventQueue-0] ERROR    at ExTest.main(ExTest.java:7)

How can I do this?

    try {
       ...
    } catch (Exception e) {
        final String s;
        ...  // &lt;-- What goes here?
        log.error( s );
    }




How to send a stacktrace to log4j?

I use log4j for logging and i want to print all logger.debug statements in a particular class / selected package.

i set the cfg as below&gt;

    log4j.category.my.pkg=info
    log4j.category.my.pkg.ab.class1=debug

but still only info messages are shown..

is this not the right way ?



log4j - set different loglevel for different packages/classes

Suddenly this keeps happening during a JUnit test. Everything was working, I wrote some new tests and this error occured. If I revert it, it won&#39;t go away. Why is that?

    log4j:WARN No appenders could be found for logger (org.springframework.test.context.junit4.SpringJUnit4ClassRunner).
    log4j:WARN Please initialize the log4j system properly.
    log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.


How to configure log4j.properties for SpringJUnit4ClassRunner?

What are the different approaches for changing the log4j log level dynamically, so that I will not have to redeploy the application. Will the changes be permanent in those cases? 

Dynamically Changing log4j log level

We are using SLF4J+Logback combination at our project for a while now and are quite happy with it, but our logging strategy is fairly simple, using straightforward class based loggers and no fancy stuff like MDC or Markers.

What I want to know is if anybody in the community actually uses these features and how they are used to improve logging/filtering.

I am specially interested in where, why and how would one use&lt;sup&gt;[1]&lt;/sup&gt; Markers for logging. They strike me as a pretty neat feature for adding semantic context into the logging - e.g. while a class may be handling multiple concerns, one may use task/concern specific markers to discriminate log statements.

What may be the best practices, conventions or strategies for creating and using markers in logging.

**Update:** I guess, what I am really after is not so much _why_ to use markers, but rather the _how_ part &amp;mdash; is there some good practices of naming markers (e.g. using plain text with spaces or dash/underscore/punctuation delimited keyword style names), should there be some sort of pool of &quot;standard names&quot;, naming stuff based on the business functions. The questions I can probably figure out for myself, but if I want to use these features systematically and introduce them to a team of developers, it makes sense to have some formalizeable set of guidelines around...

---

&lt;sup&gt;[1]&lt;/sup&gt; - By asking how to _use_ markers I am not really asking how to use API (it is really quite straight forward) - I am rather referring to the more general level of how would one set up logging around using markers consistently

Best practices for using Markers in SLF4J/Logback

How can I configure [Logback][1] to log different levels for a logger to different destinations?

For example, given the following Logback configuration, will Logback record `INFO` messages to `STDOUT` and `ERROR` messages to `STDERR`?

(Note that this example is a variation of example `logback-examples/src/main/java/chapters/configuration/sample4.xml` shown in [Chapter 3:  Logback Configuration][2]).

&lt;!-- language: xml --&gt;

    &lt;configuration&gt;
      &lt;appender name=&quot;STDOUT&quot;
       class=&quot;ch.qos.logback.core.ConsoleAppender&quot;&gt;
       &lt;encoder&gt;
         &lt;pattern&gt;
            %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
          &lt;/pattern&gt;
        &lt;/encoder&gt;
      &lt;/appender&gt;
      &lt;appender name=&quot;STDERR&quot;
       class=&quot;ch.qos.logback.core.ConsoleAppender&quot;&gt;
       &lt;encoder&gt;
         &lt;pattern&gt;
            %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
          &lt;/pattern&gt;
        &lt;/encoder&gt;
        &lt;target&gt;System.err&lt;/target&gt;
      &lt;/appender&gt;
      &lt;!-- What is the effective level of &quot;chapters.configuration&quot;? --&gt;
      &lt;logger name=&quot;chapters.configuration&quot; level=&quot;INFO&quot; additivity=&quot;false&quot;&gt;
        &lt;appender-ref ref=&quot;STDOUT&quot; /&gt;
      &lt;/logger&gt;
      &lt;logger name=&quot;chapters.configuration&quot; level=&quot;ERROR&quot; additivity=&quot;false&quot;&gt;
        &lt;appender-ref ref=&quot;STDERR&quot; /&gt;
      &lt;/logger&gt;
    
      &lt;!-- turn OFF all logging (children can override) --&gt;
      &lt;root level=&quot;OFF&quot;&gt;
        &lt;appender-ref ref=&quot;STDOUT&quot; /&gt;
      &lt;/root&gt;
    &lt;/configuration&gt;


  [1]: http://logback.qos.ch
  [2]: http://logback.qos.ch/manual/configuration.html

How can I configure Logback to log different levels for a logger to different destinations?

I&#39;ve seen several discussions on the net about how great it would be to have an XML schema or DTD for logback.xml file to have at least the very basic validation and auto-completion in IDEs like IDEA or Eclipse, but I never saw any solution.

Did you?

XML schema or DTD for logback.xml?

In our software we extensively use [**MDC**][1] to track things like session IDs and user names for web requests. This works fine while running in the original thread.

However, there&#39;s a lot of things that need to be processed in the background. For that we use the `java.concurrent.ThreadPoolExecutor` and `java.util.Timer` classes along with some self-rolled *async* execution services. All these services manage their own thread pool.

This is what [Logback&#39;s manual](http://logback.qos.ch/manual/mdc.html#managedThreads) has to say about using MDC in such an environment:

&gt; A copy of the mapped diagnostic context can not always be inherited by worker threads from the initiating thread. This is the case when java.util.concurrent.Executors is used for thread management. For instance, newCachedThreadPool method creates a ThreadPoolExecutor and like other thread pooling code, it has intricate thread creation logic.
&gt; 
&gt; In such cases, it is recommended that MDC.getCopyOfContextMap() is invoked on the original (master) thread before submitting a task to the executor. When the task runs, as its first action, it should invoke MDC.setContextMapValues() to associate the stored copy of the original MDC values with the new Executor managed thread.

This would be fine, but it is a very easy to forget adding those calls, and there is no easy way to recognize the problem until it is too late. The only sign with Log4j is that you get missing MDC info in the logs, and with Logback you get stale MDC info (since the thread in the tread pool inherits its MDC from the first task that was ran on it). Both are serious problems in a production system.

I don&#39;t see our situation special in any way, yet I could not find much about this problem on the web. Apparently, this is not something that many people bump up against, so there must be a way to avoid it. What are we doing wrong here?


  [1]: http://logback.qos.ch/manual/mdc.html

How to use MDC with thread pools?

The configuration file for logback gets found on the classpath, and is therefore Eclipse-project-specific, which is not what I want. I&#39;m using multiple Java utilities, all of them residing in a single project (this sharing the classpath), and I need to use a specific configuration for some of them.

I&#39;ve tried variable substitution and Joram configurator, but nothing worked for me. This was most probably my fault, and I&#39;m going to solve it one day, but for now I&#39;d need a simple solution.


Content Type	Original Author	Original Content on Stackoverflow
Question	pakore	View Question on Stackoverflow
Solution 1 - Java	anirvan	View Answer on Stackoverflow

Where is logback encoder pattern documentation

Java Problem Overview

Java Solutions

Solution 1 - Java

What does Html.HiddenFor do?

When is memoization automatic in GHC Haskell?

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