|
|
|
|
|
The first beta release of JDK 1.4 came out just a few weeks ago, and included with it is a new logging API. This API was first described in JSR 47. Essentially the same description is also available in the documentation of logging for the beta of JDK 1.4 .
In addition to the implementation provided in JDK 1.4, there is an open source implementation of the API for JDKs 1.2 and 1.3 called Lumberjack available on SourceForge. The library provides a way for people to begin using the API before JDK 1.4 becomes final. It will also allow them to use the API in work they do in the future, which must be done in 1.2 or 1.3 for some reason.
The logging API will be a part of the core platform in JDK 1.4, which means it will ship with the JDK. It is designed to let a Java program, servlet, applet, EJB, etc. produce messages of interest to end users, system administrators, field engineers and software developers. Especially in production situations, where things can't be run in a debugger, or doing so masks the problem that is occurring (because it is timing related, for example), such logs are frequently the greatest source (and sometimes the only source) of information about a running program.
To make sure that logging can be left in a production program, the API is designed to make logging as inexpensive as possible. To allow for code, which can produce fine grained logging, but not slow the application in normal production use, the API allows for the dynamic tuning of what log messages are produced, so that the impact of the logging code is minimized during normal operation. It also provides a number of Java interfaces and superclasses that provide "hooks" for developers to extend the API in a standard manner.
The entire logging API is contained in the package
java.util.logging and is made up of the following
interfaces and classes:
ConsoleHandler
FileHander
Filter
Formatter
Handler
Level
Logger
LoggingPermission
LogManager
LogRecord
MemoryHandler
SimpleFormatter
SocketHandler
StreamHandler
XMLFormatter
We'll examine many of these classes/interfaces in more detail below.
To help motivate the use of the logging API, we will use a simple program, based on the idea of the "Hello world" programs used to teach programming:
package com.ociweb.jnbjune2001.example1;
/**
* A simple Java Hello World program, in the tradition of
* Kernighan and Ritchie.
*/
public class HelloWorld {
public static void main(String[] args) {
HelloWorld hello = new HelloWorld("Hello world!");
hello.sayHello();
}
private String theMessage;
public HelloWorld(String message) {
theMessage = message;
}
public void sayHello() {
System.err.println(theMessage);
}
}
We will expand upon this simple program in order to demonstrate and
motivate the logging API. The first thing we will do is actually
generate a logging message. This is straightforward. We need to
create a Logger object, which represents the
HelloWorld class, like this:
package com.ociweb.jnbjune2001.example2;
import java.util.logging.Logger;
/**
* A simple Java Hello World program, in the tradition of
* Kernighan and Ritchie.
*/
public class HelloWorld {
private static Logger theLogger =
Logger.getLogger(HelloWorld.class.getName());
public static void main(String[] args) {
HelloWorld hello = new HelloWorld("Hello world!");
hello.sayHello();
}
private String theMessage;
public HelloWorld(String message) {
theMessage = message;
}
public void sayHello() {
System.err.println(theMessage);
}
}
This generates a Logger, which we can use to generate
logging messages. Since you typically use one logger per class, we
can make it a static field of the HelloWorld class. We
use the fully qualified name of the HelloWorld class as
the name of the Logger. We'll look at the reasons for
this in more detail later. Now let's actually use the
Logger an try to generate a logging message:
package com.ociweb.jnbjune2001.example3;
import java.util.logging.Logger;
/**
* A simple Java Hello World program, in the tradition of
* Kernighan and Ritchie.
*/
public class HelloWorld {
private static Logger theLogger =
Logger.getLogger(HelloWorld.class.getName());
public static void main(String[] args) {
HelloWorld hello = new HelloWorld("Hello world!");
hello.sayHello();
}
private String theMessage;
public HelloWorld(String message) {
theMessage = message;
}
public void sayHello() {
// use the 'least important' type of message, one at
// the 'finest' level.
theLogger.finest("Hello logging!");
System.err.println(theMessage);
}
}
The logging API has a configuration file and by default it provides
a handler which will send logging messages, in the form of a
LogRecord to the 'console' (System.err).
So we would expect to see the string "Hello logging!"; appear before
our message of "Hello world!". But if you run this program you only
see "Hello world!":
java com.ociweb.jnbjune2001.example3.HelloWorld
Hello world
What's going on here? The answer lies in the notion of logging levels.
Because we want to leave logging code in an application that goes
into production, we need logging to cost us very little at runtime
when it is not being used. To help achieve this, the logging system
defines a class called Level to specify the importance
of a logging record. The logging library has a set of predefined
logging levels:
| SEVERE | the highest value |
| WARNING | |
| INFO | |
| CONFIG | |
| FINE | |
| FINER | |
| FINEST | the lowest value |
In addition to these levels, there is a level ALL,
which enables logging of all records and one called
OFF that can be used to turn off logging. It is also
possible to define custom levels, but in this article we will stick
with the predefined levels.
So, now we can explain why our logging records didn't appear. We
used the finest() method of our logger, which generates
a logging record at the Level.FINEST level. By default,
only records that are at the Level.INFO level or higher
will be logged to the console. So we can cause our logging record to
appear by increasing the level of logging, like this:
package com.ociweb.jnbjune2001.example4;
import java.util.logging.Logger;
/**
* A simple Java Hello World program, in the tradition of
* Kernighan and Ritchie.
*/
public class HelloWorld {
private static Logger theLogger =
Logger.getLogger(HelloWorld.class.getName());
public static void main(String[] args) {
HelloWorld hello = new HelloWorld("Hello world!");
hello.sayHello();
}
private String theMessage;
public HelloWorld(String message) {
theMessage = message;
}
public void sayHello() {
theLogger.info("Hello logging!");
System.err.println(theMessage);
}
}
Now, when we run this program we see output like this:
java com.ociweb.jnbjune2001.example3.HelloWorld
INFO; 992149105222ms; 3207485;# 1; null; null; Hello logging!
Hello world!
Of course, using a higher logging level for a logging record is not
always what we want. Sometimes we have messages that are
finer-grained than the INFO level, and we only want
them printed when the information they contain is needed. To allow
this, the logging library allows us to control the log level of our
Loggers.
When we created our Logger, we specified the name
of our class
("com.ociweb.jnbjune2001.example4.HelloWorld") as the
Logger name, but didn't specify anything else. When a
Logger is created, it is set to screen out any record,
which doesn't have a level as high as the one the
Logger is set to have. By default, the threshold is set
to Level.INFO. This means that records we log through
our Logger will only get printed if they are at least
has high as Level.INFO. To get finer-grained records to
be logged, we need to set the logging level for our class to
something more verbose. But that isn't quite enough, because when a
record is logged to a Logger, that Logger
actually delegates the output of the record to a subclass of the
Handler class, like this:
So, even if we turn up the log level of our Logger,
when the logger hands the record to the global handlers (which it
does by default), the handlers would ignore a
Level.FINE record. To get the global handlers to
actually output the record, we need to turn up their levels, like
this:
package com.ociweb.jnbjune2001.example5;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.logging.LogManager;
/**
* A simple Java Hello World program, in the tradition of
* Kernighan and Ritchie.
*/
public class HelloWorld {
private static Logger theLogger =
Logger.getLogger(HelloWorld.class.getName());
public static void main( String[] args ) {
// The global handlers default to INFO, we have to
// crank them up. We could crank up only some of them
// if we wanted, but we will turn them all up.
Handler[] handlers =
LogManager.getLogManager().getGlobalHandlers();
for (int index = 0; index < handlers.length; index++) {
handlers[index].setLevel(Level.FINE);
}
// We also have to set our logger to log finer-grained
// messages
theLogger.setLevel(Level.FINE);
HelloWorld hello =
new HelloWorld("Hello world!");
hello.sayHello();
}
private String theMessage;
public HelloWorld(String message) {
theMessage = message;
}
public void sayHello() {
theLogger.fine("Hello logging!");
System.err.println(theMessage);
}
}
This will produce output on the console like this:
FINE; 992190676635ms; 5213611;# 1; null; null; Hello logging!
Hello world!
(For more details on the meaning of everything in this message, see the discussion of logging in the JDK 1.4 documentation)
In addition, there is another default global handler which is an
instance of the FileHandler class. It logs its records
to a location specified by a property. Instead of the simple text
format produced by SimpleFormatter (seen above), it
uses an XML format for its records. The program above produces
output like this:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE log SYSTEM "logger.dtd">
<log>
<record>
<date>2001-06-10T05:31:16</date>
<millis>992190676635</millis>
<sequence>1</sequence>
<logger>com.ociweb.jnbjune2001.example5.HelloWorld</logger>
<level>FINE</level>
<thread>5213611</thread>
<message>Hello logging!</message>
</record>
</log>
There is much more that can be done with handlers and it is even possible to create your own custom handlers. Space won't allow us to discuss them in more detail here (see the references below for places to learn more about handlers).
The LogManager is in many ways the central class of the
logging library. It provides much of the control over what gets
logged and where it gets logged, including control over
initialization of logging. In terms of logging levels, it allows us
to set different logging levels for different "areas" of the
program. To understand this, we need to think of our program as
being broken down into different areas based on the package prefix
of the classes involved. So our program areas are organized in a
tree structure, like this (only some of the tree is shown to
simplify the discussion):
But the LogManager will allow us to control things at
more than just the class level. We could set the logging level for a
package, or even a set of packages, by calling the
LogManager.setLevel(String name, Level level) method.
So, for example, we could set the logging level of all loggers for
this article to Level.FINE by making this call:
LogManager.getLogManager().setLevel("com.ociweb.jnbjune2001", Level.FINE);
This would make all classes with the package prefix of
"com.ociweb.jnbjune2001" have a level of Level.FINE:
We would still need to turn up the levels of the global handlers as before, but would not have to set the individual loggers one at a time.
In addition to providing this ability to control log levels
"en masse", the LogManager provides:
Loggers
(seen above)
Loggers (useful to
applets running in a more constrained security environment)
Level of a given package prefix or
class
While space won't permit us to go into a description of
Filters, the logging API provides them as a means to
further control what records get logged. So, if you are using
logging and find that you want use more than the log
Level of the record when deciding whether to log it,
Filters are there. They can be attached to both
Loggers and Handlers, and can be thought
of like this:
In this situation, the record must pass the Logger's
level check, the Logger's Filter, the
Handler's level check and the Handler's
Filter before it would actually be sent to the outside
world. This allows things to start out simple (with no filters) and
become more sophisticated as needed. In situations where you're
trying to get at just the right information without altering the
behavior of the program too much (and without overwhelming someone
reading the output), the ability to create more sophisticated
filters can be key to learning what needs to be known.
This article demonstrates using the fully qualified class name of
your class as the name of your Logger. This is also the
approach recommended by the Logging API documentation. However, the
LogManager and Loggers really only care
that the names used are made up of strings separated by period
characters. So if you need to organize the logging of your software
in some other manner, you can do so, as long as your scheme fits
into a tree hierarchy.
In addition, it is possible to use anonymous loggers, which don't
have a name and which don't tie into the global
Handlers. These are good for applets running in a
browser, as well as other high security situations. They can also be
used to keep classes relatively self-contained. However, when using
anonymous loggers, you don't get many of the benefits of a named
logger (the ability to have records forwarded to the global
handlers, the ability to alter logging levels of groups of loggers
by their common prefix, etc.).
The logging API seems to strike a good balance between simplicity and power. As the examples above show, it is possible to get started with logging very quickly. Yet it is designed to be extensible so that it can be tailored to your needs. This article has really just scratched the surface of what can be done with logging. In particular, topics that we didn't discuss which the API allows for include:
FiltersFiltersSocketHandler
Handlers
For more information about logging, the best sources of information
are the JDK 1.4 documentation on logging and JSR 47. There is also
Javadoc for the java.util.logging package, which
provides details about the classes.
Object Computing, Inc (OCI) has been providing educational services to clients, industries and universities since 1993. We offer one of the most comprehensive distributed Object Oriented training curricula in the country. These curricula focus on the fundamentals of OO technology; with close to 40 workshops in OOAD, Java, XML, C++/CORBA and Unix/Linux.
For further information regarding OCI's Educational Services programs, please visit our Educational Services section on the web or contact us at training.
|
|