Here's a small function that allows you to initialize JavaCall from i…

…nside Java (#90) * allow initializing from running VM to allow call-in/call-out created init_current_vm function * untabified example code * Added usage section to doc/index.md * slight wording improvement
JuliaInterop · Sep 10, 2018 · d6c3678 · d6c3678
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diff --git a/doc/index.md b/doc/index.md
@@ -52,6 +52,44 @@ julia> jcall(j_u_arrays, "binarySearch", jint, (Array{jint,1}, jint), [10,20,30,
 
 ```
 
+##Usage from a running JVM
+
+Use JNI or JNA to initialize a Julia VM, then call `JavaCall.init_current_vm()`. Here's an example using JNA:
+
+```java
+package zot.julia;
+
+import com.sun.jna.Native;
+
+public class Julia {
+    static {
+        Native.register("julia");
+        jl_init__threading();
+    }
+
+    public static double bubba = Math.random();
+
+    public static native void jl_init__threading();
+    public static native void jl_eval_string(String code);
+    public static native void jl_atexit_hook(int status);
+
+    public static void main(String args[]) {
+        System.out.println("test");
+        jl_eval_string("println(\"test from Julia\")");
+        jl_eval_string("using JavaCall");
+        jl_eval_string("JavaCall.init_current_vm()");
+        jl_eval_string("println(\"initialized VM\")");
+        jl_eval_string("jlm = @jimport java.lang.Math");
+        jl_eval_string("println(jcall(jlm, \"sin\", jdouble, (jdouble,), pi/2))");
+        jl_eval_string("jl = @jimport zot.julia.Julia");
+        System.out.println("Bubba should be " + bubba);
+        jl_eval_string("println(\"bubba: \", jfield(jl, \"bubba\", jdouble))");
+        jl_eval_string("println(\"Done with tests\")");
+        jl_atexit_hook(0);
+    }
+}
+```
+
 ##Major TODOs and Caveats
 
 *   Currently, only a low level interface is available, via `jcall`. As a result, this package is best suited for writing libraries that wrap around existing java packages. Writing user code direcly using this interface might be a bit tedious at present. A high level interface using reflection will eventually be built. 

diff --git a/src/jvm.jl b/src/jvm.jl
@@ -185,6 +185,61 @@ function init(opts)
     return
 end
 
+"""
+    init_current_vm()
+
+Allow initialization from running VM. Uses the first VM it finds.
+
+# Example using JNA
+
+```java
+package zot.julia;
+
+import com.sun.jna.Native;
+
+public class Julia {
+    static {
+        Native.register("julia");
+        jl_init__threading();
+    }
+
+    public static double bubba = Math.random();
+
+    public static native void jl_init__threading();
+    public static native void jl_eval_string(String code);
+    public static native void jl_atexit_hook(int status);
+
+    public static void main(String args[]) {
+        System.out.println("test");
+        jl_eval_string("println(\"test from Julia\")");
+        jl_eval_string("using JavaCall");
+        jl_eval_string("JavaCall.init_current_vm()");
+        jl_eval_string("println(\"initialized VM\")");
+        jl_eval_string("jlm = @jimport java.lang.Math");
+        jl_eval_string("println(jcall(jlm, \"sin\", jdouble, (jdouble,), pi/2))");
+        jl_eval_string("jl = @jimport zot.julia.Julia");
+        System.out.println("Bubba should be " + bubba);
+        jl_eval_string("println(\"bubba: \", jfield(jl, \"bubba\", jdouble))");
+        jl_eval_string("println(\"Done with tests\")");
+        jl_atexit_hook(0);
+    }
+}
+```
+"""
+function init_current_vm()
+    ppjvm = Array{Ptr{JavaVM}}(undef, 1)
+    ppenv = Array{Ptr{JNIEnv}}(undef, 1)
+    pnum = Array{Cint}(undef, 1)
+    ccall(Libdl.dlsym(libjvm, :JNI_GetCreatedJavaVMs), Cint, (Ptr{Ptr{JavaVM}}, Cint, Ptr{Cint}), ppjvm, 1, pnum)
+    global pjvm = ppjvm[1]
+    jvm = unsafe_load(pjvm)
+    global jvmfunc = unsafe_load(jvm.JNIInvokeInterface_)
+    ccall(jvmfunc.GetEnv, Cint, (Ptr{Nothing}, Ptr{Ptr{JNIEnv}}, Cint), pjvm, ppenv, JNI_VERSION_1_8)
+    global penv = ppenv[1]
+    jnienv = unsafe_load(penv)
+    global jnifunc = unsafe_load(jnienv.JNINativeInterface_) #The JNI Function table
+end
+
 function destroy()
     if (!isdefined(JavaCall, :penv) || penv == C_NULL)
         throw(JavaCallError("Called destroy without initialising Java VM"))