Jan 29, 2018

Introspection tools: Java decompilers

As you probably are already aware of, Clojure compiles down to JVM bytecode. The fact that Clojure is not an interpreted language but a compiled one makes the compilation output tangible. The resulting classes are something that we can scrutinize and understand. In this post, we will take a look at tools that help us decipher the arcane internals of JVM classfiles.

Why you need this (and what it has to do with performance)

Frankly, unless you are hacking the Clojure compiler in your free time, the information in this post is not your first priority to learn. But understanding what the Clojure code ultimately turns into gives you an extra way to improve and optimize your programs.

Decompiling the resulting code can significantly help in performance-sensitive situations where you are writing interop-heavy tight loops. Seeing whether you are really using primitive math and unboxed types is more reliable than compiler warnings. Observing how exactly the type hints influence the bytecode allows you to put only those that are necessary (and not just scatter them around until the compiler is happy).

A decompiler also gives an ability to see what some obscure Clojure macros expand into, e.g., reify, proxy, gen-class. It is difficult to see it from their implementation as it's complicated and riddled with handwritten bytecode assembly.


Let's create a simple project that we'll use to AOT-compile Clojure code. Both Boot and Leiningen variants are provided side by side, however, there isn't any substantial difference between them here.

$ boot -d seancorfield/boot-new new -n testbed   # Leiningen: lein new testbed
$ boot aot -n decomp.core target                 # Leiningen: lein update-in : assoc :aot :all -- compile
$ cd target/testbed                              # Leiningen: cd target/classes/testbed
$ ls -l
-rw-r-----  1325 core$fn__183.class
-rw-r-----   976 core$foo.class
-rw-r-----  1507 core$loading__6434__auto____181.class
-rw-r-----  2520 core__init.class

If you see the output like this, it means you have successfully compiled the sample Clojure file src/testbed/core.clj into Java classes. Before we proceed, let's understand how a single Clojure namespace produced four classfiles.

This is enough of prelude for now. In a little while, you'll be able to discover all this by yourself.

Bytecode disassembly

We can start with the simplest tool in our arsenal, javap. It is shipped with every JDK distribution and allows to disassemble the compiled classes into their bytecode representation.

Just a reminder, our sample function looks like this:

(defn foo
  "I don't do a whole lot."
  (println x "Hello, World!"))
$ javap -c core\$foo

It yields plenty of output, so we'll focus on the part where something actually happens, the invokeStatic and static initializer methods.

  public static java.lang.Object invokeStatic(java.lang.Object);
       0: getstatic     #15                 // Field const__0:Lclojure/lang/Var;
       3: invokevirtual #21                 // Method clojure/lang/Var.getRawRoot:()Ljava/lang/Object;
       6: checkcast     #23                 // class clojure/lang/IFn
       9: aload_0
      10: aconst_null
      11: astore_0
      12: ldc           #25                 // String Hello, World!
      14: invokeinterface #29,  3           // InterfaceMethod clojure/lang/IFn.invoke:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
      19: areturn

  public static {};
       0: ldc           #36                 // String clojure.core
       2: ldc           #38                 // String println
       4: invokestatic  #44                 // Method clojure/lang/RT.var:(Ljava/lang/String;Ljava/lang/String;)Lclojure/lang/Var;
       7: checkcast     #17                 // class clojure/lang/Var
      10: putstatic     #15                 // Field const__0:Lclojure/lang/Var;
      13: return

In static initializer on lines 0 and 2, we load two string constants, "clojure.core" and "println" onto the stack. Then, we call the static method clojure.lang.RT.var on them which returns a Var back on the stack. Finally, we store the resulting Var to a field const__0.

In the invokeStatic method, we begin by putting the Var from const__0 onto the stack and calling the method getRawRoot on it. This retains a function object that corresponds to Clojure's println function. aload_0 means putting the first method argument on the stack (argument x in this case).

Lines 10 and 11 might be confusing — they set the first method argument to null. This is the feature of Clojure compiler called locals clearing — it is designed to prevent retaining references to unused locals, something we might explore in another blog post.

Line 12 puts the constant "Hello, World" onto the stack. The stack now looks like this:

"Hello, World!", <x>, <println_fn>

Finally, line 14 calls the two-arg method invoke(Object,Object). So, the first two values on the stack are taken as arguments, and the third value is the object on which the invoke method is called. All three values from the stack are thus consumed, and the result of invoking the method is put back on the stack. That value is then returned with areturn.

Whew, it takes a lot of work and additional knowledge to deconstruct such a simple function! Reading JVM bytecode is similar to reading x86 assembly — you gradually get better at it, but it's still harder and slower than reading higher-level code.

Luckily, there exists an easier way to read bytecode. All we need is to decompile it to Java. Although the bytecode that the Clojure compiler produces doesn't precisely map to Java language, we can still gain a lot of insight with much less effort. It is the difference between reading Assembly and slightly hacky C code. I'd take the latter any day.

Java decompilers

There are several Java decompilers, but unfortunately, neither of them is an ultimate endgame solution, at least for Clojure. Each has shortcomings, so it might be useful to have a few of them handy, for different usecases.

Most of the time I prefer the good old JAD. It's an abandoned non-open Java decompiler written in C++, last updated in 2011. It doesn't support lambdas and some other modern Java features (not like Clojure uses them anyway). On the bright side, it is very fast, and it decompiles most Clojure bytecode quite well (beyond some convoluted try/catch constructs, but that's forgivable).

One JAD deficiency turned into a strength is that it misunderstands Clojure's above-mentioned locals clearing trick. E.g. it will decompile the function (defn foo [x y] (+ x y)) into this:

    public static Object invokeStatic(Object x, Object y) {
        return Numbers.add(x = null, y = null);

Where other decompilers will produce this:

    public static Object invokeStatic(Object x, Object y) {
        final Object o = x;
        x = null;
        final Object o2 = y;
        y = null;
        return Numbers.add(o, o2);

Now, the JAD output is not really valid — Java would execute the null assignments first, then calling add on two nulls. But after you start recognizing all the = null as locals clearing, the JAD output becomes just less noisy than the "correct" one.

Anyway, here are some other Java compilers that might or might not prove useful to you:

You don't have to install all of these to try them out. Java Decompilers Online allows you to upload your compiled Java classfile and decompile it with any of the listed tools.

Let's decompile our already existing classfile with the decompiler of your choice. I'll use JAD.

# JAD writes the output to a file named <class.jad>, we have to view it manually.
$ jad core\$foo.class && cat core\$foo.jad

package testbed;

import clojure.lang.*;

public final class core$foo extends AFunction

    public static Object invokeStatic(Object x)
        return ((IFn)const__0.getRawRoot()).invoke(x = null, "Hello, World!");

    public Object invoke(Object obj)
        obj = null;
        return invokeStatic(obj);

    public static final Var const__0 = (Var)RT.var("clojure.core", "println");

This is much better! It is evident now that clojure.core/println is stored in const__0 at initialization, and then it is called on x and "Hello, World!" when the function is invoked. With the decompiler, we can tell what's going at a glance (given that the code is trivial) where we spent minutes to the decipher bytecode.

Let's try another one:

$ jad core__init.class && cat core__init.jad

    public static void load() {
        ((IFn)new core.loading__6434__auto____181()).invoke();
        if(!((Symbol)const__1).equals(const__2)) goto _L2; else goto _L1
          goto _L3
        LockingTransaction.runInTransaction((Callable)new core.fn__183());
        const__3.bindRoot(new core.foo());

    public static void __init0() {
        const__0 = (Var)RT.var("clojure.core", "in-ns");
        const__1 = (AFn)Symbol.intern(null, "testbed.core");
        const__2 = (AFn)Symbol.intern(null, "clojure.core");
        const__3 = (Var)RT.var("testbed.core", "foo");
        const__12 = (AFn)RT.map(new Object[] {
            RT.keyword(null, "arglists"), PersistentList.create(Arrays.asList(new Object[] {
                Tuple.create(Symbol.intern(null, "x"))
            })), RT.keyword(null, "doc"), "I don't do a whole lot.", RT.keyword(null, "line"), Integer.valueOf(3), RT.keyword(null, "column"), Integer.valueOf(1), RT.keyword(null, "file"), "testbed/core.clj"

    static {
        break MISSING_BLOCK_LABEL_17;
        throw ;

What we see now is the class for the testbed.core namespace. When loaded, static initializer is called which in turn calls __init0(). That's where all constants are assigned. You can also see that's the testbed.core/foo var is created there, together with its metadata.

Afterwards, load() is called which performs some namespace-related rituals and then sets const__3 (which is our testbed.core/foo var) the proper metadata and value.

With other decompilers, you'll get a similar experience. You run the binary on the classfile and get Java code back. All decompilers have plenty of tunables that can slightly modify their behavior; just pass --help to the decompiler to see them.

Now, you are able to decompile and study any AOT-compiled Clojure bytecode. Clojure itself might be a fun place to start — you can compile it yourself or just extract the classes from downloaded clojure.jar. Then, let the decompiler loose on the classes and go wild!

Live decompilation in the REPL

One drawback of the described workflow is that it is not very dynamic. You have to save the code in a file first, then AOT-compile it with a build tool, then run the decompiler to get the results. I wanted to make it more natural to experiment and fool around, so I created clj-java-decompiler. It's a convenience wrapper around Procyon that allows decompiling any Clojure form directly from the REPL. It dramatically cuts the feedback loop and makes it much easier to explore Clojure compiler's intricacies.

Add [com.clojure-goes-fast/clj-java-decompiler "0.1.0"] to your dependencies (might as well load it dynamically) and start playing:

user> (require '[clj-java-decompiler.core :refer [decompile]])
user> (decompile (fn [] (println "Hello, decompiler!")))

// Decompiling class: user$fn__13649
import clojure.lang.*;

public final class user$fn__13649 extends AFunction
    public static final Var const__0;

    public static Object invokeStatic() {
        return ((IFn)const__0.getRawRoot()).invoke((Object)"Hello, decompiler!");

    public Object invoke() {
        return invokeStatic();

    static {
        const__0 = RT.var("clojure.core", "println");

The output is similar to what the standalone decompiler gave us, but now you can instantly decompile anything you want!

Did you know... (A few examples to whet your appetite)

...that case compiles to Java's switch?

user> (decompile (case "foo"
                   "foo" 1
                   "bar" 2

    public static Object invokeStatic() {
        final Object G__13653 = "foo";
        switch (Util.hash(G__13653)) {
            case 97299: {
                if (Util.equiv(G__13653, (Object)const__0)) {
                    return const__1;
            case 101574: {
                if (Util.equiv(G__13653, (Object)const__2)) {
                    return const__3;
        return const__4;

...that loop compiles into Java's while (and an efficient one)?

user> (decompile (loop [i 0, sum 0]
                   (if (> i 10)
                     (recur (unchecked-inc i) (unchecked-add sum i)))))

    public static Object invokeStatic() {
        long i = 0L;
        long sum = 0L;
        while (i <= 10L) {
            final long n = i + 1L;
            sum += i;
            i = n;
        return Numbers.num(sum);

...what it means when the compiler warns about reflection?

user> (decompile (fn [] (.substring @(volatile! "foobar") 3)))

    public static Object invokeStatic() {
        return Reflector.invokeInstanceMethod(((IFn)const__0.getRawRoot()).invoke(((IFn)const__1.getRawRoot()).invoke((Object)"foobar")), "substring", new Object[] { const__2 });


With the expertise obtained today, you'll be able to better understand the internals of Clojure compilation process and its runtime. A dynamic decompiler is yet another instrument under your belt that gives you a unique perspective on the code you are writing. I hope you will master this tool and use it to make your Clojure programs go even faster.