Poison

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:

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<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:

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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

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/*
* 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:

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<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
class - Java LineNumberTable: Entry explanation - Stack Overflow