TweetFollow Us on Twitter

Feb 02 Java

Volume Number: 18 (2002)
Issue Number: 02
Column Tag: Java Workshop

Java Debugging Aids

by Andrew S. Downs

Stack traces, logging, and string searching

Introduction

Many developers begin their debugging efforts very informally with a set of print statements that dump information to the console. That approach eventually leads to diminishing returns, and it becomes time to move into the realm of the interactive debugger. Or is it?

Sometimes the simplest tools are the best. Interactive debuggers are wonderful, but they invariably require a developer to spend substantial time in several areas: learning how to use the product and then applying that knowledge during a debugging session.

Arguably the most useful things that interactive debuggers provide are stack traces and variable and register values. If you know where the program is currently executing and the current program state, you can often figure out how to proceed in locating the source of a bug.

Somewhere in-between the println calls and setting of conditional breakpoints ad nauseum lies a middle ground. Determining how the program reached a certain point during execution can be done with a stack trace, which is very easy to generate from a Java exception. Since exceptions can be generated (thrown) and immediately caught, they do not need to cause the program to stop running. Being able to send that information, along with program state data, to a file or to the console allows you to run the app for awhile and follow the execution, then evaluate the data later when determining how to find and fix a bug.

This article discusses several Java utility classes that may prove useful in your debugging efforts. The first class is a simple stack trace generator that can write its output to a file or the screen. The second two are file writer classes. The fourth class contains example methods that make string searches easier.

By themselves these classes will not find bugs, but they provide information to use as a starting point in determining what to try next or where to look when problems arise. They are invasive in that you need to insert calls into your code (which can be conditionally wrapped), but they provide more details than the typical print statement, so hopefully you will find the reward worth the extra effort.

StackTrace

The first listing contains the StackTrace class. There are three types of methods here: constructors, file writers, and toString() overloads. The class works by taking an existing or new exception object, and printing out the stack trace. This has the effect of generating a stack trace at any location in your program, which is very useful for following the flow of execution. Sending the output to a file is often the most useful, and if you can easily locate certain entries in the file you will be much happier. Several of the methods in this class accept a label to accompany the trace information.

Listing 1: StackTrace.java

StackTrace
Generate a stack trace. Optionally send the text representation back to the caller, write it to a file, 
and/or include a label and timestamp.

public class StackTrace {
	// Use the native system delimiter instead of hardcoded newlines.
	private static String lineSep = 
		System.getProperty("line.separator");

	// The default output file, which can be overridden by using one of the constructors.
	private static String filename = "StackTraceLog.txt";

	// The source of the stack trace.
	Exception exceptionObject;
	
	// In the empty constructor throw and catch the exception.
	//   It keeps the caller's code fairly clean.
	StackTrace() {
		try {
			throw new Exception();
		}
		catch (Exception e) {
			exceptionObject = e;
			write(filename, this.exceptionObject);
		}
	}
	
	// This constructor allows the caller to specify the output filename. It also generates an
	//   exception for the caller.
	StackTrace(String filepath) {
		try {
			throw new Exception();
		}
		catch (Exception e) {
			exceptionObject = e;
			write(filepath, this.exceptionObject);
		}
	}
	
	// This constructor allows the caller to specify the output filename and a previously 
	//   created exception.
	StackTrace(String filepath, Exception e) {
		write(filepath, e);
	}
	
	// This constructor takes an additional string that is printed before the stack trace. 
	//   Use it to put a label on a particular trace in the file.
	StackTrace(String filepath, Exception e, String s) {
		write(filepath, e, s);
	}
	
	// This constructor trims the trace to a specified number of lines before writing.
	StackTrace(int i) {
		try {
			throw new Exception();
		}
		catch (Exception e) {
			exceptionObject = e;
			write(filename, toString(e, i));
		}
	}

	// Write to file.
	public void write(String filepath, Exception e) {
		write(filepath, e, null);
	}
	
	// Write to file: timestamp the entry and include an optional string.
	public void write(String filepath, Exception e, 
										String s) {
		try {
			java.io.FileWriter f = 
				new java.io.FileWriter(filepath, true);

			f.write((new java.util.Date()).toString() + lineSep);
			
			if ((s != null) && (s.length() > 0))
			  f.write(s + lineSep);
			
			e.printStackTrace(new java.io.PrintWriter(f, true));

			f.write(lineSep);
			f.flush();
			f.close();
		}
		catch (java.io.IOException ee) {
		}
		finally {
		}
	}
	
	// Write to file, including a timestamp.
	public void write(String filepath, String s) {
		try {
			java.io.FileWriter f = 
				new java.io.FileWriter(filepath, true);

			f.write((new java.util.Date()).toString() + lineSep);
			
			if ((s != null) && (s.length() > 0))
				f.write(s + lineSep);
			
			f.flush();
			f.close();
		}
		catch (java.io.IOException ee) {
		}
		finally {
		}
	}
	
	// The requisite override. This allows easy onscreen display by the caller.
	public String toString() {
		String retval = 
			"<StackTrace Exception attribute is null>";
		
		if (this.exceptionObject != null)
			retval = toString(this.exceptionObject);
		
		return retval;
	}
	
	// A static toString() overload for convenience. This gives the caller "one-stop 
	//   shopping", but they have to generate the exception themselves.
	public static String toString(Exception e) {
		java.io.ByteArrayOutputStream b = 
			new java.io.ByteArrayOutputStream(1024);

		e.printStackTrace(new java.io.PrintWriter(b, true));
		return b.toString();
	}
	
	// A static toString() overload that limits the number of lines returned.
	public static String toString(Exception e, int numLines) 
	{
		java.io.ByteArrayOutputStream b = 
			new java.io.ByteArrayOutputStream(1024);

		e.printStackTrace(new java.io.PrintWriter(b, true));
		String s = b.toString();
		
		StringBuffer sb = new StringBuffer(s.length());

		java.util.StringTokenizer st = 
			new java.util.StringTokenizer(s, lineSep, false);
		
		int count = 0;
		
		while (st.hasMoreElements()) {
			sb.append(st.nextElement());
			sb.append(lineSep);
			
			count++;
			if (count >= numLines)
				break;
		}
		
		return sb.toString();
	}
}

LogFile

This class uses random access files rather than streams. The first method writes a string to a file, and the second returns the contents of a newline-delimited file to the caller.

Listing 2: LogFile.java

LogFile
Write a string to a file, delete a file, and return the contents of a file.

import java.io.*;
import java.util.*;

public class LogFile {
	protected static final String mFilename = "log.txt";
	protected static final boolean mUseTimestamp = true;
	
	public static void log(String msg) {
		try {
			RandomAccessFile raf = 
				new RandomAccessFile(mFilename, "rw");
		
			raf.seek(raf.length());
		
			if (mUseTimestamp) {
				Date d = new Date();
				raf.writeBytes(d.toString() + ":" + 
					System.getProperty("line.separator"));
			}
		
			raf.writeBytes(msg + 
				System.getProperty("line.separator"));
		
			raf.close();
		}
		catch (IOException e) {
			// File error? Write to the console instead.
			System.out.println(msg + 
				System.getProperty("line.separator"));
		}
	}
	
	public static String contents() {
		String retval = "";
		
		try {
			RandomAccessFile raf = 
				new RandomAccessFile(mFilename, "rw");
		
			long length = raf.length();
			
			raf.seek(0);
		
			long i = 0;
			String s = "";
			StringBuffer sb = new StringBuffer();
			
			if (sb != null) {
				while (i < length) {
					s = raf.readLine();
					
					if (s != null) {
						i += s.length();
						sb.append(s);
					}
					else
						break;
				}
		
				retval = sb.toString();
			}
			
			raf.close();
		}
		catch (IOException e) {
			System.out.println("IOException reading logfile" + 
				System.getProperty("line.separator"));
		}
		catch (NullPointerException e) {
			System.out.println("NullPointerException reading" +  
				" logfile" + System.getProperty("line.separator"));
		}
		
		return retval;
	}
}

FileUtils

This listing contains two methods: the first writes a string to a file, the second writes a stream of bytes. There is nothing fancy here: these convenience methods simply wrap the sometimes cumbersome sequence of calls that setup, write, and close files. Note that both of these methods first delete the file if it exists. This is less useful for a logfile, where you want to retain history, but if you are replacing an existing data file with a downloaded version, then it makes sense to first remove the original file.

Listing 3: FileUtils.java

FileUtils
Write a string or byte stream to a file. The byte stream is useful when dealing with binary (non-text) 
data.

import java.io.*;

public class FileUtils {
	public static void writeFile(String s, String path,
										String file) {
		try {
			File f = new File(path + file);

			// Deleting the file is appropriate if we are replacing old data with new data.
			if (f.exists()) {
				f.delete();
				f = new File(path + file);
			}
			
			FileOutputStream fos = new FileOutputStream(f);
			OutputStreamWriter w = new OutputStreamWriter(fos);

			w.write(s, 0, s.length());
			w.close();
			fos.close();
		}
		catch (IOException e) {
			System.out.println("IOException writing string.");
		}
	}

	public static void writeFileBytes(byte s[], int start, 
										String path, String file) {
		try {
			File f = new File(path + file);
		
			if (f.exists()) {
				f.delete();
			f = new File(path + file);
			}
			
			FileOutputStream fos = new FileOutputStream(f);

			// Buffered streams often provide better performance than their non-buffered 
			//   counterparts.
			BufferedOutputStream w = 
				new BufferedOutputStream(fos);

			// The start value is useful when writing the file in pieces (e.g. lots of data). 
			w.write(s, start, s.length - start);
			w.close();
			fos.close();
		}
		catch (IOException e) {
			System.out.println("IOException writing bytes.");
		}
	}
}

Stringutils

The ability to locate substrings is one of the best things about the java.lang.String class. The problem is that often you want to do more than simply locate a string: you may want to trim portions of it at the same time. The methods presented here wrap the standard search and replace functionality. The first method locates and returns a substring. The second locates and returns multiple occurrences of the search string. The third method is similar to the second, but it trims starting at the location of a substring within the found string (such as finding attributes within an HTML tag). The last method replaces a substring.

Each of these methods receives arguments that allow you to specify whether to locate and include a second string in the returned result. For example, if you are parsing HTML tags this allows you to remove the final ‘>' from the returned string. In many situations a simpler set of methods (with fewer options) will suffice.

Listing 4: StringUtils.java

StringUtils
Various string search and replace methods.

import java.util.*;

public class StringUtils {
	public static String findSubstring(String buffer, 
		String open, String close, boolean includeFront, 
		boolean includeBack) {
		String ref = null;

		int start = buffer.indexOf(open, 0);
		int end = buffer.indexOf(close, start);

		if (start >= 0 && end >= 0 && end > start) {
			// This complicated set of conditionals checks each combination for removing 
			//   the open and close strings from the result.
			if (!includeFront) {
				if (!includeBack)
					ref = buffer.substring(start + 
						open.length(), end);
				else
					ref = buffer.substring(start + 
						open.length(), end + close.length());
			}
			else {
				if (!includeBack)
					ref = buffer.substring(start, end);
				else
					ref = buffer.substring(start, end + 
						close.length());
			}
		}

		return ref;
	}

	public static Vector findStringOccurrences(String buffer, 
		String open, String close, boolean includeOpen, 
		boolean includeClose) {
		Vector v = new Vector();

		int fromIndex = 0;
		boolean found = true;

		String ref;

		// This is similar to the one-shot findSubstring() method above. 
		//   Note the added loop that ensures we catch all occurrences of the substring 
		//   in the entire string.
		while (found && fromIndex < buffer.length()) {
			int start = buffer.indexOf(open, fromIndex);
			int end = buffer.indexOf(close, start + 
				open.length());

			if (start >= 0 && end >= 0 && end > start) {
				<snip>
				// The if-else block from findSubstring() above goes here. 
				//   The logic is the same.

				// The vector will contain the occurrences of the substring.
				v.addElement(ref.toString());

				// Update the search starting point.
				fromIndex = start + 1;
			}
			else
				found = false;
		}

		return v;
	}

	public static Vector findStringOccurrences(String buffer, 
		String open, String offsetString, String close, 
		boolean includeFront) {
		Vector v = new Vector();

		int fromIndex = 0;
		boolean found = true;

		String ref;
		
		open = open.toUpperCase();
		offsetString = offsetString.toUpperCase();
		close = close.toUpperCase();
		
		String searchString = new String(buffer);
		searchString = searchString.toUpperCase();

		while (found && fromIndex < buffer.length()) {
			int start = searchString.indexOf(open, fromIndex);
			int end = searchString.length();
			
			// Locate a substring (offsetString) within the found string.
			// Note that this adjusts the start value for the substring operation.
			start = searchString.indexOf(offsetString, 
				start + open.length());
			
			if (close.length() > 0)
				end = searchString.indexOf(close, 
					start + offsetString.length());

			if (start >= 0 && end >= 0 && end > start) {
				// The logic is simpler in this method since we do not look for a 
				//   closing string.
				if (!includeFront)
					ref = buffer.substring(start + 
						offsetString.length(), end);
				else
					ref = buffer.substring(start, end);

				v.addElement(ref);

|				fromIndex = start + 1;
			}
			else
				found = false;
		}

		return v;
	}

	public static String replaceStringOccurrence(
		String buffer, String replacement, String open, 
		String close, int occurrence, boolean includeFront) {
		StringBuffer sb = new StringBuffer();

		int fromIndex = 0;
		boolean found = true;

		// Walk through the entire buffer, looking for the i-th occurrence of the 
		//   string (the variable named open).
		while (found) {
			int start = buffer.indexOf(open, fromIndex);
			int end = buffer.indexOf(close, start + 
				open.length());

			if (start >= 0 && end >= 0 && end > start) {
				fromIndex = start + 1;

				occurrence—;
				
				// Once the count reaches zero, we have located
				//   the starting point for the replacement operation.
				if (occurrence > 0)
					continue;

				// Assemble the string from front to back.
				//   This call can also be used to simply insert the replacement string by 
				//   setting includeFront to true. 
				if (includeFront)
					sb.append(buffer.substring(0, start + 
						open.length()));
				else
					sb.append(buffer.substring(0, start));

				sb.append(replacement);
				sb.append(buffer.substring(end));
				
				break;
			}
			else
				found = false;
		}

		return sb.toString();
	}

Conclusion

The classes and methods presented here should make your logging and debugging efforts a little easier for those times when you do not require an intimate session with the debugger. The methods can be used to help determine starting points for more detailed debugging efforts.


Andrew has worked with Java since 1996. Most recently he worked on the Java desktop client and enterprise servlets for Snippets Software. You can reach him at andrew@downs.ws.

 
AAPL
$524.94
Apple Inc.
+5.93
MSFT
$40.01
Microsoft Corpora
-0.39
GOOG
$536.10
Google Inc.
-20.44

MacTech Search:
Community Search:

Software Updates via MacUpdate

Tweetbot 1.5.1 - Popular iOS twitter cli...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Mac DVDRipper Pro 4.1.7 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
PDFpenPro 6.2 - Advanced PDF toolkit for...
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Create... Read more
PDFpen 6.2 - Edit and annotate PDFs with...
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
Monolingual 1.5.9 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Starcraft II: Wings of Liberty 1.1.1.180...
Download the patch by launching the Starcraft II game and downloading it through the Battle.net connection within the app. Starcraft II: Wings of Liberty is a strategy game played in real-time. You... Read more
Sibelius 7.5.0 - Music notation solution...
Sibelius is the world's best-selling music notation software for Mac. It is as intuitive to use as a pen, yet so powerful that it does most things in less than the blink of an eye. The demo includes... Read more
Typinator 5.9 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
MYStuff Pro 2.0.16 - Create inventories...
MYStuff Pro is the most flexible way to create detail-rich inventories for your home or small business. Add items to MYStuff by dragging and dropping existing information, uploading new images, or... Read more

Latest Forum Discussions

See All

Have a Special Dead Trigger 2 Easter Bas...
Have a Special Dead Trigger 2 Easter Basket Full of Goodies, Courtesy of Madfinger Games Posted by Rob Rich on April 18th, 2014 [ permalink ] Dead Trigger 2 | Read more »
Zynga Launches Brand New Farmville Exper...
Zynga Launches Brand New Farmville Experience with Farmville 2: Country Escape Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
David. Review
David. Review By Cata Modorcea on April 18th, 2014 Our Rating: :: MINIMALISTIC IN A DIFFERENT WAYUniversal App - Designed for iPhone and iPad David is a minimalistic game wrapped inside of a soothing atmosphere in which the hero... | Read more »
Eyefi Unveils New Eyefi Cloud Service Th...
Eyefi Unveils New Eyefi Cloud Service That Allows Users to Share Media Across Personal Devices Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Tales from the Dragon Mountain: The Lair...
Tales from the Dragon Mountain: The Lair Review By Jennifer Allen on April 18th, 2014 Our Rating: :: STEADY ADVENTURINGiPad Only App - Designed for the iPad Treading a safe path, Tales from the Dragon Mountain: The Lair is a... | Read more »
Yahoo Updates Flickr App with Advanced E...
Yahoo Updates Flickr App with Advanced Editing Features and More Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
My Incredible Body - A Kid's App to...
My Incredible Body - A Kid's App to Learn about the Human Body 1.1.00 Device: iOS Universal Category: Education Price: $2.99, Version: 1.1.00 (iTunes) Description: Wouldn’t it be cool to look inside yourself and see what was going on... | Read more »
Trials Frontier Review
Trials Frontier Review By Carter Dotson on April 18th, 2014 Our Rating: :: A ROUGH LANDINGUniversal App - Designed for iPhone and iPad Trials Frontier finally brings the famed stunt racing franchise to mobile, but how much does its... | Read more »
Evernote Business Notebook by Moleskin I...
Evernote Business Notebook by Moleskin Introduced – Support Available in Evernote for iOS Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Sparkle Unleashed Review
Sparkle Unleashed Review By Jennifer Allen on April 18th, 2014 Our Rating: :: CLASSY MARBLE FLINGINGUniversal App - Designed for iPhone and iPad It’s a concept we’ve seen before, but Sparkle Unleashed is a solidly enjoyable orb... | Read more »

Price Scanner via MacPrices.net

iMacs on sale for up to $160 off MSRP this we...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
iPad Airs on sale this weekend for up to $100...
Best Buy has WiFi iPad Airs on sale for $50 off MSRP and WiFi + Cellular iPad Airs on sale for $100 off MSRP on their online store for a limited time, with prices now starting at $449. Choose free... Read more
Apple restocks refurbished Mac minis starting...
The Apple Store has restocked Apple Certified Refurbished Mac minis for up to $150 off the cost of new models. Apple’s one-year warranty is included with each mini, and shipping is free: - 2.5GHz Mac... Read more
Hyundai Brings Apple CarPlay To The 2015 Sona...
Hyundai Motor America has announced it will bring Apple CarPlay functionality to the 2015 Sonata. CarPlay is pitched as a smarter, safer and easier way to use iPhone in the car and gives iPhone users... Read more
Updated iPads Coming Sooner Than We Had Thoug...
MacRumors, cites KGI securities analyst Ming Chi Kuo, well-respected as an Apple product prognisticator, saying that Apple will introduce an upgraded iPad Air and iPad mini in 2014/Q3, meaning the... Read more
Toshiba Unveils New High And Low End Laptop M...
Toshiba has announced new laptop models covering both the high-end and low-end of the notebook computer spectrum. Toshiba 4K Ultra HD Laptop Toshiba’s new Satellite P55t features one of the world’s... Read more
Save up to $270 with Apple refurbished 13-inc...
The Apple Store has Apple Certified Refurbished October 2013 13″ Retina MacBook Pros available starting at $1099, with models up to $270 off MSRP. Apple’s one-year warranty is standard, and shipping... Read more
Apple now offering refurbished iPad mini with...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays now available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free.... Read more
Microsoft Blinks – Drops Microsoft Office 365...
Microsoft has dropped the annual subscription fee for Microsoft Office 365 Personal – which is needed in order to create and edit documents in Microsoft Office for iPad. However, Apple’s iOS and OS X... Read more
New AVG Vault Apps for iOS and Android Help K...
AVG Technologies N.V. an online security company for 177 million active users, has announced the launch of its latest mobile application, AVG Vault. The free app introduces an innovative user... Read more

Jobs Board

*Apple* Automotive Parts Department position...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive, we Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Retail - Manager - Holyoke - Apple I...
Job Summary Keeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you’re a master of them all. In the store’s fast-paced, Read more
*Apple* Retail - Manager - Apple (United Sta...
Job SummaryKeeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you're a master of them all. In the store's fast-paced, dynamic Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.