Every Java programmer is familiar with the process of inserting calls to System.out.println into troublesome code to gain insight into program behavior. Of course, once you have figured out the cause of trouble, you remove the print statements, only to put them back in when the next problem surfaces. The logging API of JDK 1.4 is designed to overcome this problem. Here are the principal advantages of the API.
Basic LoggingLet's get started with the simplest possible case. The logging system manages a default logger Logger.global that you can use instead of System.out. Use the info method to log an information message: Logger.global.info("File->Open menu item selected"); By default, the record is printed like this: May 10, 2004 10:12:15 PM LoggingImageViewer fileOpen INFO: File->Open menu item selected (Note that the time and the names of the calling class and method are automatically included.) But if you call Logger.global.setLevel(Level.OFF); at an appropriate place (such as the beginning of main), then all logging is suppressed. Advanced LoggingNow that you have seen "logging for dummies," let's go on to industrial-strength logging. In a professional application, you wouldn't want to log all records to a single global logger. Instead, you can define your own loggers. When you request a logger with a given name for the first time, it is created. Logger myLogger = Logger.getLogger("com.mycompany.myapp"); Subsequent calls to the same name yield the same logger object. Similar to package names, logger names are hierarchical. In fact, they are more hierarchical than packages. There is no semantic relationship between a package and its parent, but logger parents and children share certain properties. For example, if you set the log level on the logger "com.mycompany", then the child loggers inherit that level. There are seven logging levels:
By default, the top three levels are actually logged. You can set a different level, for example, logger.setLevel(Level.FINE); Now all levels of FINE and higher are logged. You can also use Level.ALL to turn on logging for all levels or Level.OFF to turn all logging off. There are logging methods for all levels, such as logger.warning(message); logger.fine(message); and so on. Alternatively, you can use the log method and supply the level, such as logger.log(Level.FINE, message); TIP
CAUTION
The default log record shows the name of the class and method that contain the logging call, as inferred from the call stack. However, if the virtual machine optimizes execution, accurate call information may not be available. You can use the logp method to give the precise location of the calling class and method. The method signature is void logp(Level l, String className, String methodName, String message) There are convenience methods for tracing execution flow: void entering(String className, String methodName) void entering(String className, String methodName, Object param) void entering(String className, String methodName, Object[] params) void exiting(String className, String methodName) void exiting(String className, String methodName, Object result) For example, int read(String file, String pattern) { logger.entering("com.mycompany.mylib.Reader", "read", new Object[] { file, pattern }); . . . logger.exiting("com.mycompany.mylib.Reader", "read", count); return count; } These calls generate log records of level FINER that start with the strings ENtrY and RETURN. NOTE
A common use for logging is to log unexpected exceptions. Two convenience methods include a description of the exception in the log record. void throwing(String className, String methodName, Throwable t) void log(Level l, String message, Throwable t) Typical uses are if (. . .) { IOException exception = new IOException(". . ."); logger.throwing("com.mycompany.mylib.Reader", "read", exception); throw exception; } and try { . . . } catch (IOException e) { Logger.getLogger("com.mycompany.myapp").log(Level.WARNING, "Reading image", e); } The throwing call logs a record with level FINER and a message that starts with ThrOW. Changing the Log Manager ConfigurationYou can change various properties of the logging system by editing a configuration file. The default configuration file is located at
To use another file, set the java.util.logging.config.file property to the file location by starting your application with
CAUTION
To change the default logging level, edit the configuration file and modify the line .level=INFO You can specify the logging levels for your own loggers by adding lines such as com.mycompany.myapp.level=FINE That is, append the .level suffix to the logger name. As you see later in this section, the loggers don't actually send the messages to the console that is the job of the handlers. Handlers also have levels. To see FINE messages on the console, you also need to set java.util.logging.ConsoleHandler.level=FINE CAUTION
NOTE
LocalizationYou may want to localize logging messages so that they are readable for international users. Internationalization of applications is the topic of Chapter 10 of Volume 2. Briefly, here are the points to keep in mind when localizing logging messages. Localized applications contain locale-specific information in resource bundles. A resource bundle consists of a set of mappings for various locales (such as United States or Germany). For example, a resource bundle may map the string "readingFile" into strings "Reading file" in English or "Achtung! Datei wird eingelesen" in German. A program may contain multiple resource bundles, perhaps one for menus and another for log messages. Each resource bundle has a name (such as "com.mycompany.logmessages"). To add mappings to a resource bundle, you supply a file for each locale. English message mappings are in a file com/mycompany/logmessages_en.properties, and German message mappings are in a file com/mycompany/logmessages_de.properties. (The en, de codes are the language codes.) You place the files together with the class files of your application, so that the ResourceBundle class will automatically locate them. These files are plain text files, consisting of entries such as readingFile=Achtung! Datei wird eingelesen renamingFile=Datei wird umbenannt ... When requesting a logger, you can specify a resource bundle: Logger logger = Logger.getLogger(loggerName, "com.mycompany.logmessages"); Then you specify the resource bundle key, not the actual message string, for the log message. logger.info("readingFile"); You often need to include arguments into localized messages. Then the message should contain placeholders {0}, {1}, and so on. For example, to include the file name with a log message, include the placeholder like this: Reading file {0}. Achtung! Datei {0} wird eingelesen. You then pass values into the placeholders by calling one of the following methods: logger.log(Level.INFO, "readingFile", fileName); logger.log(Level.INFO, "renamingFile", new Object[] { oldName, newName }); HandlersBy default, loggers send records to a ConsoleHandler that prints them to the System.err stream. Specifically, the logger sends the record to the parent handler, and the ultimate ancestor (with name "") has a ConsoleHandler. Like loggers, handlers have a logging level. For a record to be logged, its logging level must be above the threshold of both the logger and the handler. The log manager configuration file sets the logging level of the default console handler as java.util.logging.ConsoleHandler.level=INFO To log records with level FINE, change both the default logger level and the handler level in the configuration. Alternatively, you can bypass the configuration file altogether and install your own handler. Logger logger = Logger.getLogger("com.mycompany.myapp"); logger.setLevel(Level.FINE); logger.setUseParentHandlers(false); Handler handler = new ConsoleHandler(); handler.setLevel(Level.FINE); logger.addHandler(handler); By default, a logger sends records both to its own handlers and the handlers of the parent. Our logger is a child of the primordial logger (with name "") that sends all records with level INFO or higher to the console. But we don't want to see those records twice. For that reason, we set the useParentHandlers property to false. To send log records elsewhere, add another handler. The logging API provides two useful handlers for this purpose, a FileHandler and a SocketHandler. The SocketHandler sends records to a specified host and port. Of greater interest is the FileHandler that collects records in a file. You can simply send records to a default file handler, like this: FileHandler handler = new FileHandler(); logger.addHandler(handler); The records are sent to a file javan.log in the user's home directory, where n is a number to make the file unique. If a user's system has no concept of the user's home directory (for example, in Windows 95/98/Me), then the file is stored in a default location such as C:\Windows. By default, the records are formatted in XML. A typical log record has the form <record> <date>2002-02-04T07:45:15</date> <millis>1012837515710</millis> <sequence>1</sequence> <logger>com.mycompany.myapp</logger> <level>INFO</level> <class>com.mycompany.mylib.Reader</class> <method>read</method> <thread>10</thread> <message>Reading file corejava.gif</message> </record> You can modify the default behavior of the file handler by setting various parameters in the log manager configuration (see Table 11-2), or by using another constructor (see the API notes at the end of this section).
You probably don't want to use the default log file name. Therefore, you should use another pattern, such as %h/myapp.log. (See Table 11-3 for an explanation of the pattern variables.)
If multiple applications (or multiple copies of the same application) use the same log file, then you should turn the "append" flag on. Alternatively, use %u in the file name pattern so that each application creates a unique copy of the log. It is also a good idea to turn file rotation on. Log files are kept in a rotation sequence, such as myapp.log.0, myapp.log.1, myapp.log.2, and so on. Whenever a file exceeds the size limit, the oldest log is deleted, the other files are renamed, and a new file with generation number 0 is created. TIP
You can also define your own handlers by extending the Handler or the StreamHandler class. We define such a handler in the example program at the end of this section. That handler displays the records in a window (see Figure 11-3). Figure 11-3. A log handler that displays records in a windowThe handler extends the StreamHandler class and installs a stream whose write methods display the stream output in a text area. class WindowHandler extends StreamHandler { public WindowHandler() { . . . final JTextArea output = new JTextArea(); setOutputStream(new OutputStream() { public void write(int b) {} // not called public void write(byte[] b, int off, int len) { output.append(new String(b, off, len)); } }); } . . . } There is just one problem with this approach the handler buffers the records and only writes them to the stream when the buffer is full. Therefore, we override the publish method to flush the buffer after each record: class WindowHandler extends StreamHandler { . . . public void publish(LogRecord record) { super.publish(record); flush(); } } If you want to write more exotic stream handlers, extend the Handler class and define the publish, flush, and close methods. FiltersBy default, records are filtered according to their logging levels. Each logger and handler can have an optional filter to perform added filtering. You define a filter by implementing the Filter interface and defining the method boolean isLoggable(LogRecord record) Analyze the log record, using any criteria that you desire, and return true for those records that should be included in the log. For example, a particular filter may only be interested in the messages generated by the entering and exiting methods. The filter should then call record.getMessage() and check whether it starts with ENTRY or RETURN. To install a filter into a logger or handler, simply call the setFilter method. Note that you can have at most one filter at a time. FormattersThe ConsoleHandler and FileHandler classes emit the log records in text and XML formats. However, you can define your own formats as well. You need to extend the Formatter class and override the method String format(LogRecord record) Format the information in the record in any way you like and return the resulting string. In your format method, you may want to call the method String formatMessage(LogRecord record) That method formats the message part of the record, substituting parameters and applying localization. Many file formats (such as XML) require a head and tail part that surrounds the formatted records. In that case, override the methods String getHead(Handler h) String getTail(Handler h) Finally, call the setFormatter method to install the formatter into the handler. With so many options for logging, it is easy to lose track of the fundamentals. The "Logging Cookbook" sidebar summarizes the most common operations. Example 11-4 presents the code for the image viewer that logs events to a log window.
Example 11-4. LoggingImageViewer.java1. import java.awt.*; 2. import java.awt.event.*; 3. import java.awt.image.*; 4. import java.io.*; 5. import java.util.logging.*; 6. import javax.swing.*; 7. 8. /** 9. A modification of the image viewer program that logs 10. various events. 11. */ 12. public class LoggingImageViewer 13. { 14. public static void main(String[] args) 15. { 16. 17. if (System.getProperty("java.util.logging.config.class") == null 18. && System.getProperty("java.util.logging.config.file") == null) 19. { 20. try 21. { 22. Logger.getLogger("").setLevel(Level.ALL); 23. final int LOG_ROTATION_COUNT = 10; 24. Handler handler = new FileHandler("%h/LoggingImageViewer.log", 0, LOG_ROTATION_COUNT); 25. Logger.getLogger("").addHandler(handler); 26. } 27. catch (IOException e) 28. { 29. Logger.getLogger("com.horstmann.corejava").log(Level.SEVERE, 30. "Can't create log file handler", e); 31. } 32. } 33. 34. Handler windowHandler = new WindowHandler(); 35. windowHandler.setLevel(Level.ALL); 36. Logger.getLogger("com.horstmann.corejava").addHandler(windowHandler); 37. 38. JFrame frame = new ImageViewerFrame(); 39. frame.setTitle("LoggingImageViewer"); 40. frame.setSize(300, 400); 41. frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 42. 43. Logger.getLogger("com.horstmann.corejava").fine("Showing frame"); 44. frame.setVisible(true); 45. } 46. } 47. 48. /** 49. The frame that shows the image. 50. */ 51. class ImageViewerFrame extends JFrame 52. { 53. public ImageViewerFrame() 54. { 55. logger.entering("ImageViewerFrame", "<init>"); 56. // set up menu bar 57. JMenuBar menuBar = new JMenuBar(); 58. setJMenuBar(menuBar); 59. 60. JMenu menu = new JMenu("File"); 61. menuBar.add(menu); 62. 63. JMenuItem openItem = new JMenuItem("Open"); 64. menu.add(openItem); 65. openItem.addActionListener(new FileOpenListener()); 66. 67. JMenuItem exitItem = new JMenuItem("Exit"); 68. menu.add(exitItem); 69. exitItem.addActionListener(new 70. ActionListener() 71. { 72. public void actionPerformed(ActionEvent event) 73. { 74. logger.fine("Exiting."); 75. System.exit(0); 76. } 77. }); 78. 79. // use a label to display the images 80. label = new JLabel(); 81. add(label); 82. logger.exiting("ImageViewerFrame", "<init>"); 83. } 84. 85. private class FileOpenListener implements ActionListener 86. { 87. public void actionPerformed(ActionEvent event) 88. { 89. logger.entering("ImageViewerFrame.FileOpenListener", "actionPerformed", event); 90. 91. // set up file chooser 92. JFileChooser chooser = new JFileChooser(); 93. chooser.setCurrentDirectory(new File(".")); 94. 95. // accept all files ending with .gif 96. chooser.setFileFilter(new 97. javax.swing.filechooser.FileFilter() 98. { 99. public boolean accept(File f) 100. { 101. return f.getName().toLowerCase().endsWith(".gif") || f.isDirectory(); 102. } 103. public String getDescription() 104. { 105. return "GIF Images"; 106. } 107. }); 108. 109. // show file chooser dialog 110. int r = chooser.showOpenDialog(ImageViewerFrame.this); 111. 112. // if image file accepted, set it as icon of the label 113. if (r == JFileChooser.APPROVE_OPTION) 114. { 115. String name = chooser.getSelectedFile().getPath(); 116. logger.log(Level.FINE, "Reading file {0}", name); 117. label.setIcon(new ImageIcon(name)); 118. } 119. else 120. logger.fine("File open dialog canceled."); 121. logger.exiting("ImageViewerFrame.FileOpenListener", "actionPerformed"); 122. } 123. } 124. 125. private JLabel label; 126. private static Logger logger = Logger.getLogger("com.horstmann.corejava"); 127. } 128. 129. /** 130. A handler for displaying log records in a window. 131. */ 132. class WindowHandler extends StreamHandler 133. { 134. public WindowHandler() 135. { 136. frame = new JFrame(); 137. final JTextArea output = new JTextArea(); 138. output.setEditable(false); 139. frame.setSize(200, 200); 140. frame.add(new JScrollPane(output)); 141. frame.setFocusableWindowState(false); 142. frame.setVisible(true); 143. setOutputStream(new 144. OutputStream() 145. { 146. public void write(int b) {} // not called 147. public void write(byte[] b, int off, int len) 148. { 149. output.append(new String(b, off, len)); 150. } 151. }); 152. } 153. 154. public void publish(LogRecord record) 155. { 156. if (!frame.isVisible()) return; 157. super.publish(record); 158. flush(); 159. } 160. 161. private JFrame frame; 162. } java.util.logging.Logger 1.4
java.util.logging.Handler 1.4
java.util.logging.ConsoleHandler 1.4
java.util.logging.FileHandler 1.4
java.util.logging.LogRecord 1.4
java.util.logging.Filter 1.4
java.util.logging.Formatter 1.4
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