Problem: Fatal exception when NativeLoader loads same jar-packaged library twice

Solution: Using System.loadLibrary you can load the same library as many
times as you like because subsequent calls will be ignored. But this is
only true if you try to load the same library from the same file.
Because the NativeLoader copies the jar-packaged library to a random
directory in the temp dir we're loading the same library from different
files which causes the java process to abort fatally.

Therefore we will load the libraries with our own static library loader
wrapper that will only call the NativeLoader if that library has not
been load previously. And because we generate the wrapper in every
zproject generated jni libraray the jvm will simply pick up the first
one it encounters hence there's no need to have a common dependency
where that loader is located.

Author: Kevin Sapper

zproject_java.gsl

@@ -170,7 +170,10 @@ task generateJniHeaders(type: Exec, dependsOn: 'classes') {
             'src/main/java/$(name_path)/$(name:pascal).java'$(last ()?? ""? ",")
 .   endfor
     ]
-    commandLine("javac", "-h", "$nativeIncludes", "-classpath", "$classpath:$appclasspath", *jniClasses)
+    def utilityClasses = [
+            'src/main/java/org/zeromq/tools/ZmqNativeLoader.java'
+    ]
+    commandLine("javac", "-h", "$nativeIncludes", "-classpath", "$classpath:$appclasspath", *jniClasses, *utilityClasses)
 }
 tasks.withType(Test) {
     systemProperty "java.library.path", "/usr/lib:/usr/local/lib:$projectDir"
@@ -755,6 +758,37 @@ popd
 .endif
 .endmacro
 
+.macro generate_native_loader ()
+.   directory.create ("$(topdir)/src/main/java/org/zeromq/tools")
+.   output "$(topdir)/src/main/java/org/zeromq/tools/ZmqNativeLoader.java"
+package org.zeromq.tools;
+
+import java.util.Set;
+import java.util.HashSet;
+import org.scijava.nativelib.NativeLoader;
+
+public class ZmqNativeLoader {
+
+    private static final Set<String> loadedLibraries = new HashSet<>();
+
+    public static void loadLibrary(String libname) {
+        if (!loadedLibraries.contains(libname)) {
+            if (System.getProperty("java.vm.vendor").contains("Android")) {
+                System.loadLibrary(libname);
+            } else {
+                try {
+                    NativeLoader.loadLibrary(libname);
+                } catch (Exception e) {
+                    System.err.println("[WARN] " + e.getMessage() +" from jar. Assuming it is installed on the system.");
+                }
+            }
+            loadedLibraries.add(libname);
+        }
+    }
+
+}
+.endmacro
+
 .macro generate_class_in_java (class)
 .   directory.create ("$(topdir)/src/main/java/$(name_path)")
 .   output "$(topdir)/src/main/java/$(name_path)/$(my.class.name:pascal).java"
@@ -764,7 +798,7 @@ $(project.GENERATED_WARNING_HEADER:)
 package org.zeromq.$(project.prefix:c);
 
 import java.util.stream.Stream;
-import org.scijava.nativelib.NativeLoader;
+import org.zeromq.tools.ZmqNativeLoader;
 .   for project.use
 .       if count (project->dependencies.class, class.project = use.project) > 0
 import org.zeromq.$(use.project).*;
@@ -777,27 +811,23 @@ implements AutoCloseable\
 .   endif
 {
     static {
-        if (System.getProperty("java.vm.vendor").contains("Android")) {
-            System.loadLibrary("$(project.prefix:c)jni");
-        } else {
-            Stream.of(
+        Stream.of(
 .       for project.use
                 "$(use.prefix:c)",
 .       endfor
                 "$(project.prefix:c)"
             )
             .forEach(lib -> {
                 try {
-                    NativeLoader.loadLibrary(lib);
+                    ZmqNativeLoader.loadLibrary(lib);
                 } catch (Exception e) {
                     System.err.println("[WARN] " + e.getMessage() +" from jar. Assuming it is installed on the system.");
                 }
             });
-            try {
-                NativeLoader.loadLibrary("$(project.prefix:c)jni");
-            } catch (Exception e) {
-                System.exit (-1);
-            }
+        try {
+            ZmqNativeLoader.loadLibrary("$(project.prefix:c)jni");
+        } catch (Exception e) {
+            System.exit (-1);
         }
     }
     public long self;
@@ -990,6 +1020,7 @@ public class $(my.class.name:pascal)Test {
     project.topdir = "bindings/jni"
     directory.create (topdir)
 
+    generate_native_loader ()
     for project.class
         resolve_class (class)
         if class.okay