I recently started building a JVM, and now I have a lot of chances to see what the actual class files look like.
I could use javap, but I wanted a tool that would make the entire class file hierarchical and output it, so for practice, I made a tool that outputs the structure of the class file as JSON.
https://github.com/chgzm/cls2json Based on the Java SE 11 Edition specification.
If you generate a class file from Hello.java like the one below and feed it to the tool, JSON will be output.
Hello.java
public class Hello {
public static void main(String[] args) {
System.out.println("Hello, World.");
}
}
$ javac Hello.java
$ ./cls2json Hello.class
{"magic":"0xcafebabe","minor_version":0,"major_version":55,"constant_pool_count":29,"constant_pool":["null",{"tag":10,"class_index":6,"name_and_type_index":15},{"tag":9,"class_index":16,"name_and_type_index":17},{"tag":8,"string_index":18},{"tag":10,"class_index":19,"name_and_type_index":20},{"tag":7,"name_index":21},{"tag":7,"name_index":22},{"tag":1,"length":6,"bytes":"<init>"},{"tag":1,"length":3,"bytes":"()V"},{"tag":1,"length":4,"bytes":"Code"},{"tag":1,"length":15,"bytes":"LineNumberTable"},{"tag":1,"length":4,"bytes":"main"},{"tag":1,"length":22,"bytes":"([Ljava/lang/String;)V"},{"tag":1,"length":10,"bytes":"SourceFile"},{"tag":1,"length":10,"bytes":"Hello.java"},{"tag":12,"name_index":7,"descriptor_index":8},{"tag":7,"name_index":23},{"tag":12,"name_index":24,"descriptor_index":25},{"tag":1,"length":13,"bytes":"Hello, World."},{"tag":7,"name_index":26},{"tag":12,"name_index":27,"descriptor_index":28},{"tag":1,"length":5,"bytes":"Hello"},{"tag":1,"length":16,"bytes":"java/lang/Object"},{"tag":1,"length":16,"bytes":"java/lang/System"},{"tag":1,"length":3,"bytes":"out"},{"tag":1,"length":21,"bytes":"Ljava/io/PrintStream;"},{"tag":1,"length":19,"bytes":"java/io/PrintStream"},{"tag":1,"length":7,"bytes":"println"},{"tag":1,"length":21,"bytes":"(Ljava/lang/String;)V"}],"access_Flags":"0x33","this_class":5,"super_class":6,"interfaces_count":0,"interfaces":[],"fields_count":0,"fields":[],"methods_count":2,"methods":[{"access_flags":1,"name_index":7,"descriptor_index":8,"attributes_count":1,"attributes":[{"attribute_name_index":9,"attribute_length":29,"max_stack":1,"max_locals":1,"code_length":5,"code":[42,183,0,1,177],"exception_table_length":0,"exception_table":[],"attributes_count":1,"attributes":[{"attribute_name_index":10,"attribute_length":6,"line_number_table_length":1,"line_number_table":[{"start_pc":0,"line_number":1}]}]}]}, {"access_flags":9,"name_index":11,"descriptor_index":12,"attributes_count":1,"attributes":[{"attribute_name_index":9,"attribute_length":37,"max_stack":2,"max_locals":1,"code_length":9,"code":[178,0,2,18,3,182,0,4,177],"exception_table_length":0,"exception_table":[],"attributes_count":1,"attributes":[{"attribute_name_index":10,"attribute_length":10,"line_number_table_length":2,"line_number_table":[{"start_pc":0,"line_number":3},{"start_pc":8,"line_number":4}]}]}]}],"attributes_count":1,"attributes":[{"attribute_name_index":13,"attribute_length":2,"source_file_index":14}]}
You can see the hierarchical structure by formatting it with jq.
$ ./cls2json Hello.class | jq
{
"magic": "0xcafebabe",
"minor_version": 0,
"major_version": 55,
"constant_pool_count": 29,
"constant_pool": [
"null",
{
"tag": 10,
"class_index": 6,
"name_and_type_index": 15
},
{
"tag": 9,
"class_index": 16,
"name_and_type_index": 17
},
{
"tag": 8,
"string_index": 18
},
{
"tag": 10,
"class_index": 19,
"name_and_type_index": 20
},
{
"tag": 7,
"name_index": 21
},
{
"tag": 7,
"name_index": 22
},
{
"tag": 1,
"length": 6,
"bytes": "<init>"
},
{
"tag": 1,
"length": 3,
"bytes": "()V"
},
{
"tag": 1,
"length": 4,
"bytes": "Code"
},
{
"tag": 1,
"length": 15,
"bytes": "LineNumberTable"
},
{
"tag": 1,
"length": 4,
"bytes": "main"
},
{
"tag": 1,
"length": 22,
"bytes": "([Ljava/lang/String;)V"
},
{
"tag": 1,
"length": 10,
"bytes": "SourceFile"
},
{
"tag": 1,
"length": 10,
"bytes": "Hello.java"
},
{
"tag": 12,
"name_index": 7,
"descriptor_index": 8
},
{
"tag": 7,
"name_index": 23
},
{
"tag": 12,
"name_index": 24,
"descriptor_index": 25
},
{
"tag": 1,
"length": 13,
"bytes": "Hello, World."
},
{
"tag": 7,
"name_index": 26
},
{
"tag": 12,
"name_index": 27,
"descriptor_index": 28
},
{
"tag": 1,
"length": 5,
"bytes": "Hello"
},
{
"tag": 1,
"length": 16,
"bytes": "java/lang/Object"
},
{
"tag": 1,
"length": 16,
"bytes": "java/lang/System"
},
{
"tag": 1,
"length": 3,
"bytes": "out"
},
{
"tag": 1,
"length": 21,
"bytes": "Ljava/io/PrintStream;"
},
{
"tag": 1,
"length": 19,
"bytes": "java/io/PrintStream"
},
{
"tag": 1,
"length": 7,
"bytes": "println"
},
{
"tag": 1,
"length": 21,
"bytes": "(Ljava/lang/String;)V"
}
],
"access_Flags": "0x33",
"this_class": 5,
"super_class": 6,
"interfaces_count": 0,
"interfaces": [],
"fields_count": 0,
"fields": [],
"methods_count": 2,
"methods": [
{
"access_flags": 1,
"name_index": 7,
"descriptor_index": 8,
"attributes_count": 1,
"attributes": [
{
"attribute_name_index": 9,
"attribute_length": 29,
"max_stack": 1,
"max_locals": 1,
"code_length": 5,
"code": [
42,
183,
0,
1,
177
],
"exception_table_length": 0,
"exception_table": [],
"attributes_count": 1,
"attributes": [
{
"attribute_name_index": 10,
"attribute_length": 6,
"line_number_table_length": 1,
"line_number_table": [
{
"start_pc": 0,
"line_number": 1
}
]
}
]
}
]
},
{
"access_flags": 9,
"name_index": 11,
"descriptor_index": 12,
"attributes_count": 1,
"attributes": [
{
"attribute_name_index": 9,
"attribute_length": 37,
"max_stack": 2,
"max_locals": 1,
"code_length": 9,
"code": [
178,
0,
2,
18,
3,
182,
0,
4,
177
],
"exception_table_length": 0,
"exception_table": [],
"attributes_count": 1,
"attributes": [
{
"attribute_name_index": 10,
"attribute_length": 10,
"line_number_table_length": 2,
"line_number_table": [
{
"start_pc": 0,
"line_number": 3
},
{
"start_pc": 8,
"line_number": 4
}
]
}
]
}
]
}
],
"attributes_count": 1,
"attributes": [
{
"attribute_name_index": 13,
"attribute_length": 2,
"source_file_index": 14
}
]
}
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