Question

When working with JSP or with other languages that are converted to Java source code (or to stubs), there is often a SMAP file generated which can be later embedded to the Class file for debuggers to show better stack traces (or in case of Jasper it gets embedded automatically).

There is an old JVM bug (or RFE) to add support to include SMAP information in stack traces, but from lack of activity it seems the Sun/Oracle guys prefer that everyone post-processes his stack traces himself.

So here is my question: How to do this? Are there libraries around that do the hard work for you, or do you have to implement everything yourself?

I already found a good place where I have access to both the exception object and the class loader that loaded the "SMAP enabled" classes. Now I’d have to

• iterate over the stack trace
• Check for each entry if I can find the class
• Analyze the class with e. g. ASM to extract the SMAP info
• Write a SMAP parser that parses the reverse line mapping and the file names out of the SMAP info
• Replace the stack trace element by a new one based on the mapping (or alternatively add a new one? What is better?)
• Cache some of the info so that I don’t have to do the same stuff again if the exact same (or a similar) stack trace re-appears a few seconds later.

And since it seems to be a tedious and error-prone task, I’m hoping that someone already did this and I just have to add a library to my dependencies and call a makeStacktraceFancy method for my exceptions to make the stacktraces fancy before I log them.

Answer 1

As no one seems to know of an existing solution, I rolled my own quick&dirty one.

It does not support all SMAP features (it parses only the first stratum, and ignores vendor sections and default statum information), but it is enough for my needs.

Since the code to extract the SMAP attribute from the class is only about 50 lines, I decided to reimplement it instead of adding ASM as a dependency. The code for how to use it with ASM is in the comments.

As it is only tested very little (on a few test cases), I’ll edit the post if I encounter any severe errors.

Code is below:

/* 
 * SMAPSourceDebugExtension.java - Parse source debug extensions and
 * enhance stack traces.
 * 
 * Copyright (c) 2012 Michael Schierl
 * 
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * - Redistributions of source code must retain the above copyright notice,
 *   this list of conditions and the following disclaimer.
 *   
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 *   
 * - Neither name of the copyright holders nor the names of its
 *   contributors may be used to endorse or promote products derived from
 *   this software without specific prior written permission.
 *   
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND THE CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDERS OR THE CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
 * TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 * USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package smap;

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

/**
 * Utility class to parse Source Debug Extensions and enhance stack traces.
 * 
 * Note that only the first stratum is parsed and used.
 * 
 * @author Michael Schierl
 */
public class SMAPSourceDebugExtension {

    /**
     * Enhance a stack trace with information from source debug extensions.
     * 
     * @param t
     *            Throwable whose stack trace should be enhanced
     * @param cl
     *            Class loader to load source debug extensions from
     * @param keepOriginalFrames
     *            Whether to keep the original frames referring to Java source
     *            or drop them
     * @param packageNames
     *            Names of packages that should be scanned for source debug
     *            extensions, or empty to scan all packages
     * @throws IOException
     *             if an I/O error occurs
     */
    public static void enhanceStackTrace(Throwable t, ClassLoader cl, boolean keepOriginalFrames, String... packageNames) throws IOException {
        enhanceStackTrace(t, new HashMap<String, SMAPSourceDebugExtension>(), cl, keepOriginalFrames, packageNames);
    }

    /**
     * Enhance a stack trace with information from source debug extensions.
     * Provide a custom cache of already resolved and parsed source debug
     * extensions, to avoid parsing them for every new exception.
     * 
     * @param t
     *            Throwable whose stack trace should be enhanced
     * @param cache
     *            Cache to be used and filled
     * @param cl
     *            Class loader to load source debug extensions from
     * @param keepOriginalFrames
     *            Whether to keep the original frames referring to Java source
     *            or drop them
     * @param packageNames
     *            Names of packages that should be scanned for source debug
     *            extensions, or empty to scan all packages
     * @throws IOException
     *             if an I/O error occurs
     */
    public static void enhanceStackTrace(Throwable t, Map<String, SMAPSourceDebugExtension> cache, ClassLoader cl, boolean keepOriginalFrames, String... packageNames) throws IOException {
        StackTraceElement[] elements = t.getStackTrace();
        List<StackTraceElement> newElements = null;
        for (int i = 0; i < elements.length; i++) {
            String className = elements[i].getClassName();
            SMAPSourceDebugExtension smap = cache.get(className);
            if (smap == null) {
                boolean found = false;
                for (String packageName : packageNames) {
                    if (className.startsWith(packageName + ".")) {
                        found = true;
                        break;
                    }
                }
                if (found || packageNames.length == 0) {
                    InputStream in = cl.getResourceAsStream(className.replace('.', '/') + ".class");
                    if (in != null) {
                        String value = extractSourceDebugExtension(in);
                        in.close();
                        if (value != null) {
                            value = value.replaceAll("\r\n?", "\n");
                            if (value.startsWith("SMAP\n")) {
                                smap = new SMAPSourceDebugExtension(value);
                                cache.put(className, smap);
                            }
                        }
                    }
                }
            }
            StackTraceElement newFrame = null;
            if (smap != null) {
                int[] inputLineInfo = smap.reverseLineMapping.get(elements[i].getLineNumber());
                if (inputLineInfo != null && elements[i].getFileName().equals(smap.generatedFileName)) {
                    FileInfo inputFileInfo = smap.fileinfo.get(inputLineInfo[0]);
                    if (inputFileInfo != null) {
                        newFrame = new StackTraceElement("[" + smap.firstStratum + "]", inputFileInfo.path, inputFileInfo.name, inputLineInfo[1]);
                    }
                }
            }
            if (newFrame != null) {
                if (newElements == null) {
                    newElements = new ArrayList<StackTraceElement>(Arrays.asList(elements).subList(0, i));
                }
                if (keepOriginalFrames)
                    newElements.add(elements[i]);
                newElements.add(newFrame);
            } else if (newElements != null) {
                newElements.add(elements[i]);
            }
        }
        if (newElements != null) {
            t.setStackTrace(newElements.toArray(new StackTraceElement[newElements.size()]));
        }
        if (t.getCause() != null)
            enhanceStackTrace(t.getCause(), cache, cl, keepOriginalFrames, packageNames);
    }

    /**
     * Extract source debug extension from a class file, provided as an input
     * stream
     * 
     * @param in
     *            Input stream to read the class file
     * @return Source debug extension as a String, or <code>null</code> if none
     *         was found.
     * @throws IOException
     *             if an I/O error occurs
     */
//    // ASM version of the same method:
//    private static String extractSourceDebugExtension0(InputStream in) throws IOException {
//        ClassReader cr = new ClassReader(in);
//        final String[] result = new String[1];
//        cr.accept(new ClassVisitor(Opcodes.ASM4) {
//            @Override
//            public void visitSource(String source, String debug) {
//                result[0] = debug;
//            }
//        }, 0);
//        return result[0];
//    }
    private static String extractSourceDebugExtension(InputStream in) throws IOException {
        DataInputStream dis = new DataInputStream(in);
        boolean[] isSourceDebugExtension;
        dis.skipBytes(8);

        // read constant pool
        isSourceDebugExtension = new boolean[dis.readUnsignedShort()];
        int[] skipSizes = new int[] { 0, 0, 2, 4, 4, 0, 0, 2, 2, 4, 4, 4, 4, 2, 2, 3, 2, 2, 4 };
        for (int i = 1; i < isSourceDebugExtension.length; i++) {
            byte type = dis.readByte();
            int skipSize;
            if (type == 1) {
                String value = dis.readUTF();
                isSourceDebugExtension[i] = value.equals("SourceDebugExtension");
                skipSize = 0;
            } else if (type == 5 || type == 6) {
                skipSize = 8;
                i++;
            } else if (type > 1 && type < 19) {
                skipSize = skipSizes[type];
            } else {
                skipSize = 2;
            }
            dis.skipBytes(skipSize);
        }
        dis.skipBytes(6);
        int ifaces = dis.readUnsignedShort();
        dis.skipBytes(2 * ifaces);

        // skip fields and methods
        for (int k = 0; k < 2; k++) {
            int count = dis.readUnsignedShort();
            for (int i = 0; i < count; i++) {
                dis.skipBytes(6);
                int attrCount = dis.readUnsignedShort();
                for (int j = 0; j < attrCount; j++) {
                    dis.skipBytes(2);
                    int skip = dis.readInt();
                    dis.skipBytes(skip);
                }
            }
        }

        // read attributes and find SourceDebugExtension
        int attrCount = dis.readUnsignedShort();
        for (int i = 0; i < attrCount; i++) {
            int idx = dis.readUnsignedShort();
            int len = dis.readInt();
            if (isSourceDebugExtension[idx]) {
                byte[] buf = new byte[len];
                dis.readFully(buf);
                return new String(buf, "UTF-8");
            } else {
                dis.skipBytes(len);
            }
        }
        return null;
    }

    private final String generatedFileName, firstStratum;
    private final Map<Integer, FileInfo> fileinfo = new HashMap<Integer, FileInfo>();
    private final Map<Integer, int[]> reverseLineMapping = new HashMap<Integer, int[]>();

    private static final Pattern LINE_INFO_PATTERN = Pattern.compile("([0-9]+)(?:#([0-9]+))?(?:,([0-9]+))?:([0-9]+)(?:,([0-9]+))?");

    private SMAPSourceDebugExtension(String value) {
        String[] lines = value.split("\n");
        if (!lines[0].equals("SMAP") || !lines[3].startsWith("*S ") || !lines[4].equals("*F"))
            throw new IllegalArgumentException(value);
        generatedFileName = lines[1];
        firstStratum = lines[3].substring(3);
        int idx = 5;
        while (!lines[idx].startsWith("*")) {
            String infoline = lines[idx++], path = null;
            if (infoline.startsWith("+ ")) {
                path = lines[idx++];
                infoline = infoline.substring(2);
            }
            int pos = infoline.indexOf(" ");
            int filenum = Integer.parseInt(infoline.substring(0, pos));
            String name = infoline.substring(pos + 1);
            fileinfo.put(filenum, new FileInfo(name, path == null ? name : path));
        }
        if (lines[idx].equals("*L")) {
            idx++;
            int lastLFI = 0;
            while (!lines[idx].startsWith("*")) {
                Matcher m = LINE_INFO_PATTERN.matcher(lines[idx++]);
                if (!m.matches())
                    throw new IllegalArgumentException(lines[idx - 1]);
                int inputStartLine = Integer.parseInt(m.group(1));
                int lineFileID = m.group(2) == null ? lastLFI : Integer.parseInt(m.group(2));
                int repeatCount = m.group(3) == null ? 1 : Integer.parseInt(m.group(3));
                int outputStartLine = Integer.parseInt(m.group(4));
                int outputLineIncrement = m.group(5) == null ? 1 : Integer.parseInt(m.group(5));
                for (int i = 0; i < repeatCount; i++) {
                    int[] inputMapping = new int[] { lineFileID, inputStartLine + i };
                    int baseOL = outputStartLine + i * outputLineIncrement;
                    for (int ol = baseOL; ol < baseOL + outputLineIncrement; ol++) {
                        if (!reverseLineMapping.containsKey(ol))
                            reverseLineMapping.put(ol, inputMapping);
                    }
                }
                lastLFI = lineFileID;
            }
        }
    }

    private static class FileInfo {
        public final String name, path;

        public FileInfo(String name, String path) {
            this.name = name;
            this.path = path;
        }
    }
}