fixed error with partials when custom templates are used.

Author: HugoMario

modules/swagger-codegen/src/main/java/io/swagger/codegen/v3/templates/CodegenTemplateLoader.java

@@ -0,0 +1,30 @@
+package io.swagger.codegen.v3.templates;
+
+import com.github.jknack.handlebars.io.FileTemplateLoader;
+import org.apache.commons.lang3.StringUtils;
+
+import java.io.File;
+import java.io.IOException;
+import java.net.URL;
+
+public class CodegenTemplateLoader extends FileTemplateLoader {
+    private String templateDir;
+    private String customTemplateDir;
+
+    public CodegenTemplateLoader(String templateDir, String customTemplateDir, String suffix) {
+        super(customTemplateDir != null ? customTemplateDir : templateDir, suffix);
+        this.templateDir = templateDir;
+        this.customTemplateDir = customTemplateDir;
+    }
+
+    public URL getResource(String location) throws IOException {
+        File file = new File(location);
+        if (file.exists()) {
+            return file.toURI().toURL();
+        }
+        location = location.replace(this.customTemplateDir, StringUtils.EMPTY);
+        location = this.templateDir + location;
+
+        return this.getClass().getResource(location);
+    }
+}

modules/swagger-codegen/src/main/java/io/swagger/codegen/v3/templates/HandlebarTemplateEngine.java

@@ -2,7 +2,6 @@
 
 import com.github.jknack.handlebars.Handlebars;
 import com.github.jknack.handlebars.io.ClassPathTemplateLoader;
-import com.github.jknack.handlebars.io.FileTemplateLoader;
 import com.github.jknack.handlebars.io.TemplateLoader;
 import io.swagger.codegen.v3.CodegenConfig;
 import io.swagger.codegen.v3.CodegenConstants;
@@ -35,14 +34,13 @@ private com.github.jknack.handlebars.Template getHandlebars(String templateFile)
         final boolean needFileTemplateLoader = StringUtils.isNotBlank(config.customTemplateDir());
         final boolean fileExist = new File(templateFile).exists();
         templateFile = templateFile.replace(".mustache", StringUtils.EMPTY).replace("\\", "/");
-        final String templateDir;
-        TemplateLoader templateLoader = null;
+        final String templateDir = config.templateDir().replace("\\", "/");
+        final TemplateLoader templateLoader;
         if (needFileTemplateLoader && fileExist) {
-            templateDir = config.customTemplateDir().replace("\\", "/");
-            templateFile = resolveTemplateFile(templateDir, templateFile);
-            templateLoader = new FileTemplateLoader(templateDir, ".mustache");
+            String customTemplateDir = config.customTemplateDir().replace("\\", "/");
+            templateFile = resolveTemplateFile(customTemplateDir, templateFile);
+            templateLoader = new CodegenTemplateLoader("/" + templateDir, customTemplateDir, ".mustache");
         } else {
-            templateDir = config.templateDir().replace("\\", "/");
             templateFile = resolveTemplateFile(templateDir, templateFile);
             templateLoader = new ClassPathTemplateLoader("/" + templateDir, ".mustache");
         }