code: private static void generateInstance(String className) {
  		builder.addField(FieldSpec.builder(className(className), "instance", Modifier.PRIVATE, Modifier.STATIC).addJavadoc("instance of shared preferences\n").build());
  		// instance
  		MethodSpec.Builder methodBuilder = MethodSpec.methodBuilder("getInstance").addModifiers(Modifier.PUBLIC, Modifier.STATIC, Modifier.SYNCHRONIZED)
  				.addJavadoc("get instance of shared preferences\n").returns(className(className));
  
  		methodBuilder.beginControlFlow("if (instance==null)");
  		methodBuilder.addStatement("instance=new $L()", className(className));
  		methodBuilder.addComment("read and write instance to sync with default values");
  		methodBuilder.addStatement("instance.write(instance.read())");
  		methodBuilder.endControlFlow();
  		methodBuilder.addCode("return instance;\n");
  
  		builder.addMethod(methodBuilder.build());
  	}
docs: Generate instance of shared preferences.
  
  @param className
  the class name