fix generate java-inflector crash issue on windows (OpenAPITools#6011)

Author: chenqping

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

@@ -82,7 +82,7 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
     protected String licenseUrl = "http://unlicense.org";
     protected String projectFolder = "src/main";
     protected String projectTestFolder = "src/test";
-    protected String sourceFolder = projectFolder + "/java";
+    protected String sourceFolder = projectFolder + File.separator +"java";
     protected String testFolder = projectTestFolder + "/java";
     protected boolean fullJavaUtil;
     protected boolean discriminatorCaseSensitive = true; // True if the discriminator value lookup should be case-sensitive.

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/GroovyClientCodegen.java

@@ -60,7 +60,7 @@ public GroovyClientCodegen() {
         languageSpecificPrimitives.add("File");
         languageSpecificPrimitives.add("Map");
 
-        sourceFolder = projectFolder + "/groovy";
+        sourceFolder = projectFolder + File.separator +"groovy";
         outputFolder = "generated-code/groovy";
         modelTemplateFiles.put("model.mustache", ".groovy");
         apiTemplateFiles.put("api.mustache", ".groovy");

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaCXFClientCodegen.java

@@ -54,7 +54,7 @@ public JavaCXFClientCodegen() {
 
         supportsInheritance = true;
 
-        sourceFolder = "src/gen/java";
+        sourceFolder = "src"+ File.separator +"gen"+ File.separator +"java";
         invokerPackage = "org.openapitools.api";
         artifactId = "openapi-jaxrs-client";
         dateLibrary = "legacy"; //TODO: add joda support to all jax-rs

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaInflectorServerCodegen.java

@@ -26,6 +26,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.io.File;
 import java.util.*;
 
 import static org.openapitools.codegen.utils.StringUtils.camelize;
@@ -43,7 +44,7 @@ public JavaInflectorServerCodegen() {
 
         modifyFeatureSet(features -> features.includeDocumentationFeatures(DocumentationFeature.Readme));
 
-        sourceFolder = "src/gen/java";
+        sourceFolder = "src"+ File.separator+"gen"+ File.separator +"java";
         apiTestTemplateFiles.clear(); // TODO: add test template
         embeddedTemplateDir = templateDir = "JavaInflector";
         invokerPackage = "org.openapitools.controllers";