One of the latest fashions in the Java world is GraalVM. For someone who has been around, I still remember the "Write once, run anywhere" slogan of Java virtual machine. Apparently, the wheel has spun back, now people want to write native code in Java, which has to be compiled to different machine codes in different platforms.

The driving forces for this change may include the often lamented slow startup time of Java programs. For the brave new world of serverless functions, a quick starting program makes a lot of economic sense. The small but vibrant Clojure community, which I consider myself a part of, is particularly excited about this new found capability of JVM, for we now have one more way to write quick starting command line programs using our beloved language. The other way is to use ClojureScript on one of the Javascript engines, but Javascript engines are slower than JVM and are not as nice. For example, Babashka is one such example that has taken the community by storm.

As the author of Datalevin, a relatively new open source Datalog database, I have decided to answer the users' call for a native version of Datalevin using GraalVM native image technology. After some trials and errors, I have gotten a native version of Datalevin to pass all the tests. Now I can share some experience.

Failed attempts to compile Datalevn to native image

The difficulty of directly compiling Datalevin to GraalVM native image lies in our use of JNR library to wrap LMDB, the underlying key-value store that is written in C. As far as I know, no one has succeeded in getting JNR to work with native image. This is not for the lack of trying, see for example this effort and this issue. I myself have tried really hard. Although I think I have gone further than many, the need to rerun some class initialization at runtime prevented me from succeeding. Apparently, some GraalVM team members also think it is not easy to do at this point.

I am left with two options: one is to use a different LMDB wrapper that does not use JNR. For example, a JNI based LMDB wrapper is available and GraalVM native image does support JNI. However, that code has not been updated for a long time and its maintainer seems to have joined LMDBJava, the LMDB wrapper that uses JNR. The other option is to write a LMDB wrapper of my own that can run in native image. So I did just that.

Writing C code in Java/Clojure

As a native technology, GraalVM obviously has the facility to interface with C code. Not just that, it must also have the facility to write native code on its own. Fortunately, such facility is also packaged as a SDK that is publicly available. To use this SDK, one has to be familiar with not just Java, but also C programming, because effectively, it is writing low level C code in Java syntax.

The only problem is that there isn't an official documentation for this kind of programming, just a Java doc and a coding example. After some research, I also found a few other examples: an OpenGL demo, a neo4j native driver and an uname utility. I hope that this blog post adds to this growing library of GraalVM specific programming examples.

Import C data and functions

Our goal is to write a LMDB wrapper with GraalVM SDK without using JNI.

The first step is to make Java aware of the LMDB C data structures and functions declared in the header file lmdb.h, so that our Java code can use them. This step is fairly easy and pleasant, compared with JNI. All one needs to do is to write a Java class to list all those C structs, enums and functions as native interfaces and annotate these with appropriate GraalVM specific annotations.

For example, for a C struct definition:

typedef struct MDB_val {
    size_t       mv_size;   /**< size of the data item */
    void        *mv_data;   /**< address of the data item */
} MDB_val;

The corresponding Java declaration is the following:

    public interface MDB_val extends PointerBase {

        long get_mv_size();

        void set_mv_size(long value);

        VoidPointer get_mv_data();

        void set_mv_data(VoidPointer value);

The CStruct annotation tells Java which C struct to import. I gave the Java interface the same name as C so it is easier for me to keep track. One could also name it otherwise, e.g. "MDBVal" if one wants to follow CamelCases. It does not matter.

PointerBase interface indicates a native word type, and is the root of all C pointers being imported into Java. Most things in GraalVM SDK work with these native word types.

CField annotation imports a C struct field as a Java method. Setters and getters need to be declared separately. Again, I give these methods similar names as C field names, but you may prefer to follow Java name convention instead, e.g. "getMvSize".

VoidPointer is a faithful translation of void *.

For an opaque C struct declaration that does not list its fields, one must add an isIncomplete option to the annotation, otherwise compilation will fail with a "sizeOf" related error. For example, for a C struct like this:

typedef struct MDB_env MDB_env;

The corresponding Java declaration is this:

    @CStruct(value = "MDB_env", isIncomplete = true)
    public interface MDB_env extends PointerBase {}

C functions are directly translated into Java static native methods, e.g.:

int  mdb_env_get_maxkeysize(MDB_env *env);

The corresponding Java declaration is the following:

    public static native int mdb_env_get_maxkeysize(MDB_env env);

Notice that the pointer to MDB_env struct is replaced by MDB_env Java interface declared above, as it extends PointerBase.

For parameters with double pointer type, one can declare them as WordPointer in Java, as they point to another pointer (i.e. word). For example, this C function:

int  mdb_env_create(MDB_env **env);

is translated into this Java code:

    public static native int mdb_env_create(WordPointer envPtr);

Finally, for these annotations to work, the containing class must be annotated with appropriate C context, which is often defined as a static inner class of the class, like so:

public final class Lib {

    public static final class Directives implements CContext.Directives {
        public List<String> getHeaderFiles() {
            return Collections.singletonList("<lmdb.h>");

        public List<String> getLibraries() {
            return Arrays.asList("lmdb");

The CContext.Directives class imports the necessary C header file and finds the corresponding C library. In this particular example, we are importing the system wide LMDB header file and library installed by apt install liblmdb-dev on Debian/Ubuntu. An example of importing local header file and library can be found in Datalevin source code here.

The code of all C to Java translations is here

Clojure specific considerations

Clojure deftype supports Java annotations, so that is what I used to implement the higher level constructs of the LMDB wrapper. For example:

(deftype ^{Retention RetentionPolicy/RUNTIME
           CContext  {:value Lib$Directives}}
    LMDB [^Env env
          ^String dir
          ^RtxPool pool
          ^ConcurrentHashMap dbis
          ^:volatile-mutable closed?]

One limitation that one needs to be aware of when writing native image related Clojure code, is that most things in the GraalVM SDK inherit from org.graalvm.word.WordBase, not from java.lang.Object, which breaks the hidden assumption of a lot of Clojure constructs. For example, one cannot do this:

(let [env ^Lib$MDB_env (Env/create)]

Because Lib$MDB_env extends PointerBase, but Clojure let seems to assume everything getting bound is an Object. An "expecting Object but got Word" or vice visa error will be thrown during compilation for these cases. So writing a thin layer of wrapper for these GraalVM word types seems to be inevitable, unless Clojure can be enhanced to be more native image programming friendly.

Another point of caution is about arrays. GraalVM uses reflection to create array objects, but if these array objects are not declared at build time, the code won't run. For instance, for Clojure into-array function, we should not omit the optional first argument for element data type, otherwise, one has to manually specify the array type in the reflection.json file during compilation, or the Graal runtime will complain about "Class such and such is instantiated reflectively but was never registered". For Clojure dynamically generated classes with names such as these, "datalevin.test.query$fn__12734$fn__12739$fn__12740[]", it is impossible to add them manually in reflection.json. So the only solution is to specify the element data type in code, e.g. (into-array Object data), instead of (into-array data).

The full Clojure project for native Datalevin is here.

Memory management and pointer wrangling

The main challenge of building a LMDB wrapper is to find a way to put transaction data into and get query data out of LMDB. As shown above, LMDB use a MDB_val struct to represents input/output data. All it contains is a data size and a pointer to the data. LMDBJava uses JNR and Unsafe or reflections to manipulate a java.nio.ByteBuffer to achieve this. Since we cannot use these in this project, we have to come up with a GraalVM specific solution.

It turned out the code to do this is quite easy to write. Instead of allocating the ByteBuffer in Java and presenting it to C, we manage the memory in C and present it as a ByteBuffer in Java, without all that Unsafe and reflection shenanigans.

 * Wrap LMDB MDB_val to look like a ByteBuffer at the Java side
public class BufVal {

    private ByteBuffer inBuf;

    private VoidPointer data;
    private Lib.MDB_val ptr;

    public BufVal(int size) {

        data = UnmanagedMemory.calloc(size);
        ptr = UnmanagedMemory.calloc(SizeOf.get(Lib.MDB_val.class));


        inBuf = CTypeConversion.asByteBuffer(data, size);

     * Set the limit of internal ByteBuffer to the current position, and update
     * the MDB_val size to be the same, so no unnecessary bytes are written
    public void flip() {

     * Set the limit of internal ByteBuffer to capacity, and update
     * the MDB_val size to be the same, so it is ready to accept writes
    public void clear() {

     * Free memory
    public void close() {;;

     * Return a ByteBuffer for getting data out of MDB_val
    public ByteBuffer outBuf() {
        ByteBuffer buf = CTypeConversion.asByteBuffer(ptr.get_mv_data(),
        return buf;

     * Reset MDB_val pointer back to internal ByteBuffer, and return it
     * for putting data into MDB_val
    public ByteBuffer inBuf() {
        return inBuf;

     * Return the MDB_val pointer to be used in LMDB calls
    public Lib.MDB_val getVal() {
        return (Lib.MDB_val)ptr;

     * factory method to create an instance
    public static BufVal create(int size) {
        return new BufVal(size);

We allocate the memory for the data and the MDB_val struct with the UnmanagedMemory.calloc static method from the SDK. This allocates memory from the heap just like in C. We will then need to free the memory ourselves.

If the memory is needed only for a short period of time, the other options are PinnedObject or StackValue classes of the SDK. The former allows creating Java objects and then pinning them down, so that the garbage collector does not move them or delete them, in order to get a stable pointer to work with at the C side. The latter allocates objects from the stack so it is short-lived. These cases do not fit our need for long term pointers to database data structures, so we use UnmanagedMemory.

The SDK utility CTypeConversion.asByteBuffer static method is what makes our effort possible. We can simply expose a MDB_val as a ByteBuffer to Java in the constructor for incoming data, and in outBuf() for outgoing data. The rest of the code is just bookkeeping for the ByteBuffer.


I am happy that this effort is turning out well. The GraalVM SDK is quite pleasant to use, once one figures it out. I wish that this API can be made available in regular JVM as well, so we can truly write it once, and use it everywhere, regardless the underlying languages and platforms.

Important Update (2023-01-06)

For GraalVM versions newer than 21.3.0, those C context annotations in the Clojure code shown above should be removed, because CContext is now marked as HOST_ONLY, and is available only during build time. Basically, only those Java classes that directly wrap the C code should now add these annotations.


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