diff --git a/README.md b/README.md
index c8b9c7428..03527c9d7 100644
--- a/README.md
+++ b/README.md
@@ -154,7 +154,8 @@ not generate anything.
 
 Each Zserio extension should be packed in a single jar file.
 
-All Zserio extensions which are available on the Java classpath are automatically loaded during Zserio compiler
+All Zserio extensions which are available on the Java classpath or which are located in the directory
+from which the Zserio jar executable has been run, are automatically loaded during Zserio compiler
 startup.
 
 More information how to implement a new Zserio extension can be found in the
diff --git a/ant_task/src/zserio/ant/ToolWrapper.java b/ant_task/src/zserio/ant/ToolWrapper.java
index 9de359010..c0a720947 100644
--- a/ant_task/src/zserio/ant/ToolWrapper.java
+++ b/ant_task/src/zserio/ant/ToolWrapper.java
@@ -1,5 +1,6 @@
 package zserio.ant;
 
+import java.io.File;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.MalformedURLException;
@@ -40,25 +41,16 @@ public ToolWrapper(String className, Iterable<Path> classPath, boolean ignoreErr
             String [] files = p.list();
             for (String f : files)
             {
-                String u = null;
                 try
                 {
-                    if (f.endsWith(".jar"))
-                    {
-                        u = "jar:file:"+f+"!/";
-                    }
-                    else
-                    {
-                        u = "file:"+f+"/";
-                    }
-
-                    System.out.println("Adding " + u + " to classpath");
-                    urls.add(new URL(u));
+                    final URL pathUrl = new File(f).toURI().toURL();
+                    System.out.println("Adding " + pathUrl + " to classpath");
+                    urls.add(pathUrl);
 
                 }
-                catch (MalformedURLException e)
+                catch (MalformedURLException | RuntimeException e)
                 {
-                    throw new BuildException("Malformed URL: " + u);
+                    throw new BuildException("Malformed URL from file: " + f + " (" + e + ")");
                 }
             }
         }
diff --git a/compiler/core/build.xml b/compiler/core/build.xml
index 04dddd2e4..36cdde263 100644
--- a/compiler/core/build.xml
+++ b/compiler/core/build.xml
@@ -81,7 +81,7 @@ spotbugs.home_dir       - Location of the spotbugs tool. If not set, spotbugs is
         <attribute name="package" default="zserio.antlr"/>
         <sequential>
             <echo message="Generating ANTLR 4 grammar: @{target}"/>
-            <java classname="org.antlr.v4.Tool" failonerror="true">
+            <java classname="org.antlr.v4.Tool" failonerror="true" fork="true">
                 <classpath>
                     <pathelement location="${3rdparty.jar_dir}/${3rdparty.antlr4.jar_file_name}"/>
                 </classpath>
diff --git a/compiler/core/src/zserio/tools/ExtensionManager.java b/compiler/core/src/zserio/tools/ExtensionManager.java
index 0d0534366..f0d7682d0 100644
--- a/compiler/core/src/zserio/tools/ExtensionManager.java
+++ b/compiler/core/src/zserio/tools/ExtensionManager.java
@@ -4,11 +4,13 @@
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.io.StringWriter;
-import java.io.UnsupportedEncodingException;
 import java.net.MalformedURLException;
+import java.net.URI;
+import java.net.URISyntaxException;
 import java.net.URL;
 import java.net.URLClassLoader;
-import java.net.URLDecoder;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Iterator;
@@ -105,11 +107,11 @@ public void callExtensions(Root rootNode, ExtensionParameters parameters) throws
     private ClassLoader getClassLoader()
     {
         final ClassLoader currentClassLoader = getClass().getClassLoader();
-        final String workingDirectory = getWorkingDirectory();
+        final File workingDirectory = getWorkingDirectory();
         if (workingDirectory == null)
             return currentClassLoader;
 
-        final File[] fileList = new File(workingDirectory).listFiles();
+        final File[] fileList = workingDirectory.listFiles();
         if (fileList == null)
             return currentClassLoader;
 
@@ -120,7 +122,7 @@ private ClassLoader getClassLoader()
             {
                 if (isFileZserioExtension(file))
                 {
-                    urlArray.add(new URL("file:" + file.getPath()));
+                    urlArray.add(file.toURI().toURL());
                     urlArray.addAll(getDependentJarsFromManifest(file));
                 }
             }
@@ -136,17 +138,17 @@ private ClassLoader getClassLoader()
         return urlClassLoader;
     }
 
-    private String getWorkingDirectory()
+    private File getWorkingDirectory()
     {
         try
         {
-            final String execPath = getClass().getProtectionDomain().getCodeSource().getLocation().getPath();
-            final String decodedExecPath = URLDecoder.decode(execPath, "UTF-8");
-            final String workingDirectory = new File(decodedExecPath).getParent();
+            final URI execUri = getClass().getProtectionDomain().getCodeSource().getLocation().toURI();
+            final Path execPath = Paths.get(execUri);
+            final Path execParentPath = execPath.getParent();
 
-            return workingDirectory;
+            return (execParentPath == null) ? null : execParentPath.toFile();
         }
-        catch (SecurityException | UnsupportedEncodingException excpt)
+        catch (SecurityException | URISyntaxException excpt)
         {
             return null;
         }
diff --git a/test/core/build.xml b/test/core/build.xml
index 778f102f2..a0ebf77b9 100644
--- a/test/core/build.xml
+++ b/test/core/build.xml
@@ -123,9 +123,10 @@ spotbugs.home_dir               - Location of the spotbugs tool. If not set, spo
             <zserio srcPath="${@{testName}.zs.in_dir}" srcFile="@{zsFile}" ignoreError="@{ignoreErrors}"
                     extraArgs="${zserio.extra_args}">
                 <classpath>
-                    <fileset dir="${zserio.jar_dir}">
-                        <include name="*.jar"/>
-                    </fileset>
+                    <file name="${zserio.jar_dir}/zserio_core.jar"/>
+                    <file name="${zserio.jar_dir}/antlr-4.7.2-complete.jar"/>
+                    <file name="${zserio.jar_dir}/commons-cli-1.4.jar"/>
+                    <file name="${zserio.jar_dir}/freemarker-2.3.28.jar"/>
                 </classpath>
                 <args/>
             </zserio>
diff --git a/test/extensions/build.xml b/test/extensions/build.xml
index 4f3fbced7..dc06b31a0 100644
--- a/test/extensions/build.xml
+++ b/test/extensions/build.xml
@@ -138,9 +138,10 @@ spotbugs.home_dir               - Location of the spotbugs tool. If not set, spo
             <zserio srcPath="${@{testName}.zs.in_dir}" srcFile="@{zsFile}" ignoreError="@{ignoreErrors}"
                     extraArgs="${zserio.extra_args}">
                 <classpath>
-                    <fileset dir="${zserio.jar_dir}">
-                        <include name="*.jar"/>
-                    </fileset>
+                    <file name="${zserio.jar_dir}/zserio_core.jar"/>
+                    <file name="${zserio.jar_dir}/antlr-4.7.2-complete.jar"/>
+                    <file name="${zserio.jar_dir}/commons-cli-1.4.jar"/>
+                    <file name="${zserio.jar_dir}/freemarker-2.3.28.jar"/>
                 </classpath>
                 <arg name="java" value="${@{testName}.zs.out_dir}"/>
                 <args/>