ronaldweinland.infon or printf debug statements. log4j offers a hierarchical way . A disadvantage could be, that due to the io instructions used, it is slightly slower. support in a project or would like to get a complete project done, feel free to contact us. ronaldweinland.info log4j Tutorial in PDF - Learn how log4j framework works starting from The " complete log4j manual" documents version of the log4j logging framework.
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You are authorized to download one copy of the electronic book entitled "The complete log4j Manual" and associated software written by Ceki Gülcü, hereafter . No introduction would be complete without the customary Hello, World example. See the Lookups manual page for more details. The. ronaldweinland.info log4j is a reliable, fast and flexible logging framework (APIs) written in Java, which is distributed.
With the aid of introductory material and many examples, new users should quickly come up to speed. Seasoned log4j users will also find fresh material not discussed anywhere else. Advanced log4j topics are also covered in detail so that the reader can harness the full power of log4j. Readers are very much encouraged to enter a review login required. We were faced with a very aggressive deadline, and needed answers fast. This book was very well written with lots of examples, and let me find what I needed very quickly and move on with development.
No knowledge of class loaders is necessary as SLF4J does not make use nor does it directly access any class loaders. Given the simplicity of the SLF4J interfaces and its deployment model, developers of new logging frameworks should find it very easy to write SLF4J bindings.
Libraries Authors of widely-distributed components and libraries may code against the SLF4J interface in order to avoid imposing an logging framework on their end-user. Thus, the end-user may choose the desired logging framework at deployment time by inserting the corresponding slf4j binding on the classpath, which may be changed later by replacing an existing binding with another on the class path and restarting the application.
This approach has proven to be simple and very robust. As of SLF4J version 1.
In order to avoid imposing a logging framework on the end-user, Wombat's distribution includes slf4j-api. Even in the absence of any SLF4J binding on the class path, Wombat's distribution will still work out-of-the-box, and without requiring the end-user to download a binding from SLF4J's web-site.
Only when the end-user decides to enable logging will she need to install the SLF4J binding corresponding to the logging framework chosen by her.
Basic rule Embedded components such as libraries or frameworks should not declare a dependency on any SLF4J binding but only depend on slf4j-api. When a library declares a transitive dependency on a specific binding, that binding is imposed on the end-user negating the purpose of SLF4J. Note that declaring a non-transitive dependency on a binding, for example for testing, does not affect the end-user.
SLF4J usage in embedded components is also discussed in the FAQ in relation with logging configuration , dependency reduction and testing. Declaring project dependencies for logging Given Maven's transitive dependency rules, for "regular" projects not libraries or frameworks declaring logging dependencies can be accomplished with a single dependency declaration. In addition to logback-classic Note that explicitly declaring a dependency on logback-core In addition to slf4j-log4j Note that explicitly declaring a dependency on log4j In addition to slf4j-jdk Note that explicitly declaring a dependency on slf4j-api From the client's perspective all versions of slf4j-api are compatible.
After these two verifications, log4j creates a LoggingEvent 7 object, log4j's internal representation of log requests. We talk about a logging event when discussing log4j internals, whereas we use the term logging request to refer to the invocation of log4j printing methods by the user. Consider the two terms as quasi-synonyms used interchangeably in the text.
Some of the fields composing a LoggingEvent object are assigned within the object constructor. These fields are the level of the request, the logger, the current time, the message parameter passed by the user and the associated throwable if any. The current time is a value returned by System. This value is locale independent. Ignoring drifts in their respective clocks, two logging events generated at the same instant on two computers in different time zones, possibly thousands of kilometers apart, will bear the same timestamp.
LocationInformation is log4j's internal representation of the caller's location which includes the caller's file name, line number and class name. The location information is extracted from the program execution stack in a relatively slow and time consuming process. Moreover, location information may not always be available because certain just-in-time compilers and other code optimizers modify the structure of the execution stack.
LoggingEvent is serializable class. This allows a logging event instance created on one machine to be logged remotely on a different host. The remote host can manipulate a deserialized event as if it were generated locally. Reading the source code of the LoggingEvent class you perhaps noticed that several of its fields are marked public which is contrary to object oriented design principles.
If you look more carefully, you will notice that several of these fields are marked as final public allowing any class to access these fields directly but not to modify them. For various and involved technical reasons, the level field is marked as transient public. However, LoggingEvent objects are only visible to appenders attached to loggers in the hierarchy or to associated layouts.
In theory, a rogue appender could modify the logger or level of an event. So far this has never been a problem although a malicious appender or layout could take advantage of this vulnerability. It is hard to imagine an exploit based on this vulnerability but one can never be completely sure. In any case, make sure to verify the origin of any appender used in a sensitive application. In future log4j releases, the level field will be marked as private, only accessible through accessor methods.
Performance One of the often-cited arguments against logging is its computational cost. This is a legitimate concern as even moderately sized applications can generate thousands of log requests. Much effort was spent measuring and tweaking logging performance. Log4j claims to be reliable, fast and extensible in that order of priority.
Independently of these efforts, the user should still be aware of the following performance issues. Logging performance when logging is turned off entirely.
You can turn off logging entirely by setting the threshold of a repository to Level. OFF, the highest possible level.
See Hierarchy-wide Threshold on page 14 on how to set a threshold of a repository. When logging is turned off entirely or for a level below the threshold, the cost of a log request consists of a method invocation plus an integer comparison.
However, any method invocation involves the "hidden" cost of parameter construction. For example, for some logger x writing, x. The cost of parameter construction can be quite high and depends on the size of the parameters involved. To avoid the cost of parameter construction you can write: if x.
On the other hand, if the logger is debug-enabled, it will twice incur the cost of evaluating whether the logger is enabled or not: once in isdebugenabled and once in debug. If a method contains multiple log statements, it may be possible to factor out the tests. Logger is a class and not an interface. This measurably reduces the cost of method invocation at the cost of some flexibility, although in some recent JVMs, the performance difference became negligible.
Certain users resort to preprocessing or compile-time techniques to compile out all log statements. Most java compilers, including javac and jikes, will remove conditional statements which are assured to always evaluate as false. In the next example, the compiler will remove the dead if statement in the foo method by compiling it as an immediately returning method.
Note that if the D static variable were not final, the compiler could not have optimized the if statement. One can conveniently place them in a single class and import it in other classes.
As long as the conditional expression is guaranteed to be false, the compiler will eliminate dead if statements. Section of the Java Language specification, entitled Unreachable Statements, requires that every java compiler carry out conservative flow analysis to make sure all statements are reachable.
Compilers are required to report an error if a statement cannot be executed because it is unreachable. Interestingly enough, if statements are a special case such that unreachable if statements do not generate compile time errors, in contrast to other unreachable statements.
In fact, the authors of the specification explicitly state that this behavior is required in order to support conditional compilation. The same section also warns that "conditionally compilation" has significant impact on binary compatibility. For example, if classes A, B, and C import a flag variable form class F, then changing the value of the flag variable and compiling F will not impact the already compiled versions of A, B, and C.
Beware of this problem if your classes are compiled selectively. However, since the resulting application binary does not contain any log statements, logging cannot be turned on for that binary.
This is perhaps a disproportionate price to pay in exchange for a possibly small performance gain. The performance gain will be significant only if log statements are placed in tightloops where the same log request is invoked potentially millions or even billions of times.
Inserting logging statements in tight-loops is a lose-lose proposal. It will slow down your application even if logging is turned off or generate massive and hence useless logging output if enabled. The performance of deciding whether to log or not to log when logging is turned on.
This is essentially the performance of walking the logger hierarchy. When logging is turned on, log4j still needs to compare the level of the log request with the level of the request logger.
However, loggers may not have an assigned level; they can inherit them from the logger hierarchy. Thus, before inheriting a level, the logger may need to search its ancestors. There has been a serious effort to make this hierarchy walk to be as fast as possible.
For example, child loggers link only to their existing ancestors. This significantly improves the speed of the walk, especially in "sparse" hierarchies. The cost of walking the hierarchy is typically 3 times slower than just checking whether logging is turned off entirely.
Actual logging formatting and writing to the output device. This is the cost of formatting the log output and sending it to its target destination. Here again, a serious effort was made to make layouts formatters perform as quickly as possible. The same is true for appenders.
The typical cost of actually logging is about to microseconds. See org. Although feature-rich, one of the foremost design goals of log4j was speed of execution, a requirement which is second only to reliability. Some log4j components have been rewritten many times to improve performance. Nevertheless, contributors frequently come up with new optimizations.
Now that you have an understanding of loggers, their hierarchical nature, of levels, appenders, layouts and other log4j building blocks, the next chapter will show you to configure log4j declaratively using configuration scripts.
This considerably accelerates the output rate on the console. Configuration scripts In symbols one observes an advantage in discovery which is greatest when they express the exact nature of a thing briefly and, as it were, picture it; then indeed the labor of thought is wonderfully diminished. Leibniz Inserting log requests into the application code requires a fair amount of planning and effort. My observations show that approximately 4 percent of code is dedicated to logging.
Consequently, even moderately sized applications will have thousands of logging statements embedded within their source code. Given their number, it becomes imperative to manage these log statements without the need to modify them manually.
The log4j environment is fully configurable programmatically. However, it is far more flexible to configure log4j using configuration files. Simplest approach using BasicConfigurator As mentioned in Chapter 1, the simplest way to configure log4j is by using Basic- Configurator. Let us give a taste of how this is done with the help of an imaginary application called MyApp1.
It then defines a static logger variable with the name chapter3. This variant of the getlogger method takes a class parameter. The returned logger will have the fully qualified class name of the class parameter. MyApp1 uses the Foo class defined in the same package, as listed below. This method is hardwired to add a ConsoleAppender to the root logger. The output of the command java chapter3. Refer to the section entitled Running the Examples on page 2 for more details.
The figure below depicts the object diagram of MyApp1 after just having called the BasicConfigurator.
As a side note, let me mention that in log4j child loggers link only to their existing ancestors. In particular, the logger named chapter3. This noticeably improves the performance of hierarchy walks and also reduces log4j's memory footprint by a small amount. The MyApp1 class configures log4j by invoking BasicConfigurator. All other classes only need to import the org. For example, the only dependence of the Foo class on log4j is the org.
Except code that configures log4j if such code exists user code does not need to depend on log4j except for the Logger class. Given that the java. Fortunately, it is easy to modify MyApp1 so that the log output can be controlled at run-time. Here is a slightly modified version called MyApp2. Example BasicConfigurator. The output will reflect this difference. It is often very useful to define the log4j.
As in: java -Dlog4j. Another way to instruct log4j to print internal debugging messages is to define the log4j.
As in: log4j. Internal log4j messages only appear on the console. As of this writing, the internal debug messages cannot be redirected to output devices other than the console.
The limitation stems from the fact that log4j cannot use itself to perform its own logging. This can be considered an intriguing architectural flaw which we intend to address in future versions of log4j. Fortunately enough, it seems that this limitation has not had any practical impact. Configuration files are fed to a PropertyConfigurator instance which parses them and configures log4j accordingly.
A sample configuration file reproducing the BasicConfigurator. More interesting and useful examples will be given shortly. However, before delving into examples, a more formal definition of the property file format is in order.
Armed with the knowledge about the expected syntax, you will be able to define elaborate configuration files of your own. In the syntax definitions below constant width italic elements represent replaceable elements supplied by the user.
Elements between brackets represent optional elements. Setting the repository-wide threshold The repository-wide threshold filters logging requests by level, regardless of the logger.
The syntax is: log4j. A custom level 9 value can be specified in the form level classname. The quote characters are not required and must be omitted in actual configuration files, as illustrated in the following examples. The following directive disables all logging for the entire hierarchy. In other words, the hierarchy-wide threshold is inactive by default, letting all logging requests to pass through to the next filter. Appender configuration Appenders are named entities.
Although discouraged, appender names can contain dots which do not pos- 9 We shall discuss custom levels in detail in Chapter 8 Extending log4j.
The first step in configuring an appender is to specify its name and class: Specify the appender name as well its class.
The next step is to set the options of the appender. Any setter method taking a single primitive java type, an Integer, a Long, a String or a Boolean parameter corresponds to an option property.
For example, given that the FileAppender class contains setappend boolean , setbuffersize int and setfile string as member methods, then it follows that Append, BufferSize and File are all valid option names.
Log4j can also deal with setter methods taking a parameter of type org. For example, since the AppenderSkeleton class 10 has setthreshold level as a member method, Threshold is a valid option for all log4j appenders extending AppenderSkeleton. Thus, even without a formal list for the options of a given appender, it is easy to discover these options by looking at the setter methods of the appender and the setter methods of its superclasses.
For each named appender you can also configure its layout. The syntax for configuring a layout for a given named appender is shown next.
In contrast to appenders which are named, layouts do not have names as they do not need to be addressed individually. A layout is associated with one and only one appender.
In the most typical case, appenders are attached to the root logger. The syntax for configuring the root logger is: log4j.
A custom level value can be specified in the form level classname. The quote characters are not required and must be omitted in actual configuration files. If a level value is specified, then the root level is set to the corresponding level. If no level value is specified, then the level of the root logger remains untouched. Multiple appenders can be attached to any logger, including the root logger.
Each named appender mentioned in the root logger directive will be added to the root logger. However, before adding these appenders, all the appenders previously attached to root logger are closed and then detached.
For non-root categories the syntax is almost the same: log4j. There are no restrictions on logger names. These values have the effect of setting the logger s level to null. If no level value is supplied, then the level of the named logger remains untouched.
By default loggers inherit their level from the hierarchy. Similar to the root logger syntax, each named appender will be attached to the name logger. The syntax for setting the additivity flag of a logger is: log4j.
There is a rationale for this idiosyncrasy. By design all logger names are considered valid, in particular a name that ends with. The additivity flag applies only to non-root loggers because a root logger, placed at the top of the hierarchy by construction, has no parent loggers. ObjectRenderers Object renderers, introduced on page 19, allow you to customize the way message objects of a given type are converted to string before being logged.
This is done by specifying an ObjectRenderer for the object type would like to customize. The syntax for specifying object renderers is as follows. The following directive instructs log4j to apply the com. The syntax of variable substitution is similar to that of Unix shells.
The value of the substituted variable can be defined as a system property or in the configuration file itself.
The value of the key is first searched in the system properties, and if not found there, it is then searched in the configuration file being parsed. For example, if java.
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