Examining Different Java Compilers

I have three JDKs on my system and I'd like to see if I can tell if they produce different .class files when compiling .java sources and if I can tell which one produced which .class file. The JDKs on my system are:

$ dpkg -l '*jdk*' | grep ^ii | cut -d' ' -f3
openjdk-11-jdk-headless:amd64
openjdk-8-jdk-headless:amd64
oracle-java8-jdk

For each of these compilers, I want to do:

mkdircompiler-foo&&compiler-foo
echo"public class Test { }">Test.java
javacTest.java
javap-verboseTest.class>Test.javap

With a bit of BASH foo, this becomes:

find/usr/lib/jvm/-namejavac|
whileread-rjava_compiler;do
compiler=$(basename"$(dirname"$(dirname"${java_compiler}")")")
fn=${compiler//-/}
mkdir"${fn}"
echo"public class Test { }">"${fn}"/Test.java
"${java_compiler}""${fn}"/Test.java
"$(dirname"${java_compiler}")"/javap-verbose"${fn}"/Test.class\
&>"${fn}"/Test.javap
done

Since I had three compilers, I now have three directories, each with its own set of Test files:

$ tree
.
├── java11openjdkamd64
│  ├── Test.class
│  ├── Test.java
│  └── Test.javap
├── java8openjdkamd64
│  ├── Test.class
│  ├── Test.java
│  └── Test.javap
└── oraclejava8jdkamd64
 ├── Test.class
 ├── Test.java
 └── Test.javap
3 directories, 9 files

Now, on to the fun part, diffing. There's no difference between the .class files generated by Oracle JDK 8 and OpenJDK 8:

$ diff java8openjdkamd64/Test.class oraclejava8jdkamd64/Test.class
$ echo $?
0

The only difference is the path to the source file as reported by javap:

$ diff java8openjdkamd64/Test.javap oraclejava8jdkamd64/Test.javap
1c1
< Classfile /tmp/examine/java8openjdkamd64/Test.class
---
> Classfile /tmp/examine/oraclejava8jdkamd64/Test.class

On the other hand, when examining the difference in the .class files produced by OpenJDK 8 and OpenJDK 11, there are several difference. First off, the .class files differ:

$ diff java8openjdkamd64/Test.class java11openjdkamd64/Test.class
Binary files java8openjdkamd64/Test.class and java11openjdkamd64/Test.class differ

javap has more details for us:

$diffjava8openjdkamd64/Test.javapjava11openjdkamd64/Test.javap

1,3c1,3
< Classfile /home/torstein/tmp/examine/java8openjdkamd64/Test.class
< Last modified 04-Dec-2018; size 182 bytes
< MD5 checksum f22f52551c287057ed6d62d392d5647e
---
> Classfile /home/torstein/tmp/examine/java11openjdkamd64/Test.class
> Last modified 4 Dec 2018; size 182 bytes
> MD5 checksum 893ccf6fab86d0488097a4052360d7c1
7,8c7,11
< major version: 52
< flags: ACC_PUBLIC, ACC_SUPER
---
> major version: 55
> flags: (0x0021) ACC_PUBLIC, ACC_SUPER
> this_class: #2 // Test
> super_class: #3 // java/lang/Object
> interfaces: 0, fields: 0, methods: 1, attributes: 1
25c28
< flags: ACC_PUBLIC
---
> flags: (0x0001) ACC_PUBLIC

The difference here that really tells us that these are two compilers defaulting to different Java language versions is the major version field:

< major version: 52
---
> major version: 55

Pretty interesting, eh?

~ /home 🏠 ~ talks 💬 ~ bash ~ craftsmanship ~ db ~ dongxi ~ emacs ~ escenic ~ iam ~ java ~ js ~ language ~ latex ~ ldap ~ life ~ linux ~ llm ~ mac-os-x ~ mt-foo ~ network ~ norsk ~ python ~ quotes ~ running ~ security ~ travel ~ unix ~ various ~ vcs ~ webdesign ~ windows ~ discoveries ~ cv 🧙 ~

Licensed under CC BY Creative Commons License ~ 📡 RSS feed ~ ✉ torstein.k.johansen @ gmail ~ 🐘 @skybert@hachyderm.io ~ 🎥 youtube.com/@skybert