LineNumberTable

今天在调试时又遇到行号无法对齐的问题，因为之前开发 Java Agent 时遇到过，常见于源码 jar 与运行时 jar 不一致导致，本文简要记录。

调试的类为 javax.servlet.annotation.MultipartConfig，即最常见的 Servlet API 中的类，比如对以下四个字段打上了断点：

你会发现断点停在了类上的注释处，查询应用中对 Servlet API 的依赖为：

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.1.0</version>
    <scope>provided</scope>
</dependency>

说明 IDE 是使用的 javax.servlet-api 对应的 jar 包内容进行显示，而我们知道运行时是使用的 Tomcat lib 目录中的 servlet-api.jar，于是我将该 jar 解压，并执行命令 javap -v MultipartConfigElement.class，输出如下：

Classfile /private/tmp/javax/servlet/MultipartConfigElement.class
  Last modified Jun 3, 2020; size 1574 bytes
  MD5 checksum ddf46ee87acaf5c92faa4d9b022352a3
  Compiled from "MultipartConfigElement.java"
public class javax.servlet.MultipartConfigElement
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #14.#45        // java/lang/Object."<init>":()V
   #2 = Fieldref           #13.#46        // javax/servlet/MultipartConfigElement.location:Ljava/lang/String;
   #3 = String             #47            //
   #4 = Long               -1l
   #6 = Fieldref           #13.#48        // javax/servlet/MultipartConfigElement.maxFileSize:J
   #7 = Fieldref           #13.#49        // javax/servlet/MultipartConfigElement.maxRequestSize:J
   #8 = Fieldref           #13.#50        // javax/servlet/MultipartConfigElement.fileSizeThreshold:I
   #9 = InterfaceMethodref #51.#52        // javax/servlet/annotation/MultipartConfig.location:()Ljava/lang/String;
  #10 = InterfaceMethodref #51.#53        // javax/servlet/annotation/MultipartConfig.maxFileSize:()J
  #11 = InterfaceMethodref #51.#54        // javax/servlet/annotation/MultipartConfig.maxRequestSize:()J
  #12 = InterfaceMethodref #51.#55        // javax/servlet/annotation/MultipartConfig.fileSizeThreshold:()I
  #13 = Class              #56            // javax/servlet/MultipartConfigElement
  #14 = Class              #57            // java/lang/Object
  #15 = Utf8               location
  #16 = Utf8               Ljava/lang/String;
  #17 = Utf8               maxFileSize
  #18 = Utf8               J
  #19 = Utf8               maxRequestSize
  #20 = Utf8               fileSizeThreshold
  #21 = Utf8               I
  #22 = Utf8               <init>
  #23 = Utf8               (Ljava/lang/String;)V
  #24 = Utf8               Code
  #25 = Utf8               LineNumberTable
  #26 = Utf8               LocalVariableTable
  #27 = Utf8               this
  #28 = Utf8               Ljavax/servlet/MultipartConfigElement;
  #29 = Utf8               StackMapTable
  #30 = Class              #56            // javax/servlet/MultipartConfigElement
  #31 = Class              #58            // java/lang/String
  #32 = Utf8               (Ljava/lang/String;JJI)V
  #33 = Utf8               (Ljavax/servlet/annotation/MultipartConfig;)V
  #34 = Utf8               annotation
  #35 = Utf8               Ljavax/servlet/annotation/MultipartConfig;
  #36 = Utf8               getLocation
  #37 = Utf8               ()Ljava/lang/String;
  #38 = Utf8               getMaxFileSize
  #39 = Utf8               ()J
  #40 = Utf8               getMaxRequestSize
  #41 = Utf8               getFileSizeThreshold
  #42 = Utf8               ()I
  #43 = Utf8               SourceFile
  #44 = Utf8               MultipartConfigElement.java
  #45 = NameAndType        #22:#59        // "<init>":()V
  #46 = NameAndType        #15:#16        // location:Ljava/lang/String;
  #47 = Utf8
  #48 = NameAndType        #17:#18        // maxFileSize:J
  #49 = NameAndType        #19:#18        // maxRequestSize:J
  #50 = NameAndType        #20:#21        // fileSizeThreshold:I
  #51 = Class              #60            // javax/servlet/annotation/MultipartConfig
  #52 = NameAndType        #15:#37        // location:()Ljava/lang/String;
  #53 = NameAndType        #17:#39        // maxFileSize:()J
  #54 = NameAndType        #19:#39        // maxRequestSize:()J
  #55 = NameAndType        #20:#42        // fileSizeThreshold:()I
  #56 = Utf8               javax/servlet/MultipartConfigElement
  #57 = Utf8               java/lang/Object
  #58 = Utf8               java/lang/String
  #59 = Utf8               ()V
  #60 = Utf8               javax/servlet/annotation/MultipartConfig
{
  public javax.servlet.MultipartConfigElement(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_1
         5: ifnull        16
         8: aload_0
         9: aload_1
        10: putfield      #2                  // Field location:Ljava/lang/String;
        13: goto          22
        16: aload_0
        17: ldc           #3                  // String
        19: putfield      #2                  // Field location:Ljava/lang/String;
        22: aload_0
        23: ldc2_w        #4                  // long -1l
        26: putfield      #6                  // Field maxFileSize:J
        29: aload_0
        30: ldc2_w        #4                  // long -1l
        33: putfield      #7                  // Field maxRequestSize:J
        36: aload_0
        37: iconst_0
        38: putfield      #8                  // Field fileSizeThreshold:I
        41: return
      LineNumberTable:
        line 32: 0
        line 34: 4
        line 35: 8
        line 37: 16
        line 39: 22
        line 40: 29
        line 41: 36
        line 42: 41
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      42     0  this   Ljavax/servlet/MultipartConfigElement;
            0      42     1 location   Ljava/lang/String;
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 16
          locals = [ class javax/servlet/MultipartConfigElement, class java/lang/String ]
          stack = []
        frame_type = 5 /* same */

  public javax.servlet.MultipartConfigElement(java.lang.String, long, long, int);
    descriptor: (Ljava/lang/String;JJI)V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=7, args_size=5
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_1
         5: ifnull        16
         8: aload_0
         9: aload_1
        10: putfield      #2                  // Field location:Ljava/lang/String;
        13: goto          22
        16: aload_0
        17: ldc           #3                  // String
        19: putfield      #2                  // Field location:Ljava/lang/String;
        22: aload_0
        23: lload_2
        24: putfield      #6                  // Field maxFileSize:J
        27: aload_0
        28: lload         4
        30: putfield      #7                  // Field maxRequestSize:J
        33: iload         6
        35: ifle          47
        38: aload_0
        39: iload         6
        41: putfield      #8                  // Field fileSizeThreshold:I
        44: goto          52
        47: aload_0
        48: iconst_0
        49: putfield      #8                  // Field fileSizeThreshold:I
        52: return
      LineNumberTable:
        line 45: 0
        line 47: 4
        line 48: 8
        line 50: 16
        line 52: 22
        line 53: 27
        line 56: 33
        line 57: 38
        line 59: 47
        line 61: 52
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      53     0  this   Ljavax/servlet/MultipartConfigElement;
            0      53     1 location   Ljava/lang/String;
            0      53     2 maxFileSize   J
            0      53     4 maxRequestSize   J
            0      53     6 fileSizeThreshold   I
      StackMapTable: number_of_entries = 4
        frame_type = 255 /* full_frame */
          offset_delta = 16
          locals = [ class javax/servlet/MultipartConfigElement, class java/lang/String, long, long, int ]
          stack = []
        frame_type = 5 /* same */
        frame_type = 24 /* same */
        frame_type = 4 /* same */

  public javax.servlet.MultipartConfigElement(javax.servlet.annotation.MultipartConfig);
    descriptor: (Ljavax/servlet/annotation/MultipartConfig;)V
    flags: ACC_PUBLIC
    Code:
      stack=3, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: aload_1
         6: invokeinterface #9,  1            // InterfaceMethod javax/servlet/annotation/MultipartConfig.location:()Ljava/lang/String;
        11: putfield      #2                  // Field location:Ljava/lang/String;
        14: aload_0
        15: aload_1
        16: invokeinterface #10,  1           // InterfaceMethod javax/servlet/annotation/MultipartConfig.maxFileSize:()J
        21: putfield      #6                  // Field maxFileSize:J
        24: aload_0
        25: aload_1
        26: invokeinterface #11,  1           // InterfaceMethod javax/servlet/annotation/MultipartConfig.maxRequestSize:()J
        31: putfield      #7                  // Field maxRequestSize:J
        34: aload_0
        35: aload_1
        36: invokeinterface #12,  1           // InterfaceMethod javax/servlet/annotation/MultipartConfig.fileSizeThreshold:()I
        41: putfield      #8                  // Field fileSizeThreshold:I
        44: return
      LineNumberTable:
        line 63: 0
        line 64: 4
        line 65: 14
        line 66: 24
        line 67: 34
        line 68: 44
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      45     0  this   Ljavax/servlet/MultipartConfigElement;
            0      45     1 annotation   Ljavax/servlet/annotation/MultipartConfig;

  public java.lang.String getLocation();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #2                  // Field location:Ljava/lang/String;
         4: areturn
      LineNumberTable:
        line 71: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ljavax/servlet/MultipartConfigElement;

  public long getMaxFileSize();
    descriptor: ()J
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: getfield      #6                  // Field maxFileSize:J
         4: lreturn
      LineNumberTable:
        line 75: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ljavax/servlet/MultipartConfigElement;

  public long getMaxRequestSize();
    descriptor: ()J
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: getfield      #7                  // Field maxRequestSize:J
         4: lreturn
      LineNumberTable:
        line 79: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ljavax/servlet/MultipartConfigElement;

  public int getFileSizeThreshold();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #8                  // Field fileSizeThreshold:I
         4: ireturn
      LineNumberTable:
        line 83: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ljavax/servlet/MultipartConfigElement;
}
SourceFile: "MultipartConfigElement.java"

从以上的字节码信息中可以看出 line 48 正在执行 public javax.servlet.MultipartConfigElement(java.lang.String, long, long, int) 方法中对 location 字段的相关操作，很明显，与 javax.servlet-api 源码不匹配。查询 Tomcat 对应版本的源码，很容易发现 lib 目录中的 servlet-api.jar 由 Tomcat 中的源码构建而来，源码位于 MultipartConfigElement.java at 8.5.56:

/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements.  See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License.  You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package javax.servlet;

import javax.servlet.annotation.MultipartConfig;

/**
 * @since Servlet 3.0
 * TODO SERVLET3 - Add comments
 */
public class MultipartConfigElement {

    private final String location;// = "";
    private final long maxFileSize;// = -1;
    private final long maxRequestSize;// = -1;
    private final int fileSizeThreshold;// = 0;

    public MultipartConfigElement(String location) {
        // Keep empty string default if location is null
        if (location != null) {
            this.location = location;
        } else {
            this.location = "";
        }
        this.maxFileSize = -1;
        this.maxRequestSize = -1;
        this.fileSizeThreshold = 0;
    }

    public MultipartConfigElement(String location, long maxFileSize,
            long maxRequestSize, int fileSizeThreshold) {
        // Keep empty string default if location is null
        if (location != null) {
            this.location = location;
        } else {
            this.location = "";
        }
        this.maxFileSize = maxFileSize;
        this.maxRequestSize = maxRequestSize;
        // Avoid threshold values of less than zero as they cause trigger NPEs
        // in the Commons FileUpload port for fields that have no data.
        if (fileSizeThreshold > 0) {
            this.fileSizeThreshold = fileSizeThreshold;
        } else {
            this.fileSizeThreshold = 0;
        }
    }

    public MultipartConfigElement(MultipartConfig annotation) {
        location = annotation.location();
        maxFileSize = annotation.maxFileSize();
        maxRequestSize = annotation.maxRequestSize();
        fileSizeThreshold = annotation.fileSizeThreshold();
    }

    public String getLocation() {
        return location;
    }

    public long getMaxFileSize() {
        return maxFileSize;
    }

    public long getMaxRequestSize() {
        return maxRequestSize;
    }

    public int getFileSizeThreshold() {
        return fileSizeThreshold;
    }
}

其中 48 行与我们反编译的字节码文件匹配。明确原因后，我将工程里的依赖调整为：

<dependency>
    <groupId>org.apache.tomcat</groupId>
    <artifactId>tomcat-servlet-api</artifactId>
    <version>8.5.56</version>
    <scope>provided</scope>
</dependency>

调整后断点暂停的位置正常。我比较了 JavaEE 与 Tomcat 中关于 servlet-api 的类，发现其主要的不同在于注释部分，导致行号无法对上，JavaEE 中 MultipartConfigElement.java 的源码可以参考：servlet-spec/MultipartConfigElement.java at 3.1.0。

Reference

Chapter 4. The class File Format - 4.7.12. The LineNumberTable Attribute
Java LineNumberTable: Entry explanation - Stack Overflow