Custom service does not work using API JSON

Issue

• We developed a custom service using service builder. The service method receives a string as parameter.
• We access it through JSON API.
• When the string length is short (some hundred of kilobytes), the service works correctly.
• But when the string length is longer (some megabytes -p.e. 4 or 5-) we get following error in log:

[JSONWebServiceServiceAction:114] Unable to deserialize object

Environment

• Liferay DXP 7.2
• Tomcat 9.0.33 application server.

Resolution

The analysis:

• Firstly, following configuration would be added to get additional information about the error.
  • Set next property in portal-ext.properties file to true:

json.service.serialize.throwable=true

• Restart the server, and add next log category with DEBUG value:

com.liferay.portal.jsonwebservice.JSONWebServiceServiceAction

• Now, Liferay DXP's log should show following trace:

2022-11-21 13:57:41.055 DEBUG [http-nio-8080-exec-2][JSONWebServiceServiceAction:111] Unable to deserialize object
java.lang.IllegalStateException: Unable to deserialize object
        at com.liferay.portal.json.JSONFactoryImpl.looseDeserialize(JSONFactoryImpl.java:229)
        at com.liferay.portal.kernel.json.JSONFactoryUtil.looseDeserialize(JSONFactoryUtil.java:103)
        at com.liferay.portal.jsonwebservice.action.JSONWebServiceInvokerAction.invoke(JSONWebServiceInvokerAction.java:84)
        at com.liferay.portal.jsonwebservice.JSONWebServiceServiceAction.getJSON(JSONWebServiceServiceAction.java:68)
        at com.liferay.portal.struts.JSONAction.execute(JSONAction.java:74)
        at com.liferay.portal.servlet.JSONServlet.service(JSONServlet.java:63)
        at com.liferay.portal.jsonwebservice.JSONWebServiceServlet.service(JSONWebServiceServlet.java:63)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
....Caused by: jodd.json.JsonException: Syntax error! Invalid char: c_offset: 0 near: "cmd=%7B%22..." [Sanitized]
        at jodd.json.JsonParser.syntaxError(JsonParser.java:1012)
        at jodd.json.JsonParser.parseValue(JsonParser.java:421)
        at jodd.json.JsonParser._parse(JsonParser.java:271)
        at jodd.json.JsonParser.parse(JsonParser.java:240)
        at com.liferay.portal.json.JSONDeserializerImpl.deserialize(JSONDeserializerImpl.java:41)
        at com.liferay.portal.json.JSONFactoryImpl.looseDeserialize(JSONFactoryImpl.java:222)
        ... 75 more

• The root cause is focused in how the application server manages the POST request. Tomcat has a maximum value for POST size requests. By default, this limit is 2 megabytes.
• If this limit is over, the parameters' values will not be returned by Tomcat.
• Liferay DXP checks the returned parameters, and if they are null it gets the JSON object contained into the request:

public JSONWebServiceInvokerAction(HttpServletRequest httpServletRequest) {
    _httpServletRequest = httpServletRequest;

    String command = httpServletRequest.getParameter(Constants.CMD);

    if (command == null) {
        try {
            command = ServletUtil.readRequestBody(httpServletRequest);
        }
        catch (IOException ioException) {
            throw new IllegalArgumentException(ioException);
        }
    }

    _command = command;
}

• Parameter CMD starts with the 'cmd=' string, so it is not a valid JSON object and therefore it causes the parsing error:

Caused by: jodd.json.JsonException: Syntax error! Invalid char: c_offset: 0 near: "cmd=%7B%22..." 

The solution

• So, the solution is to configure Tomcat application server with a higher value for POST size requests in HTTP Connector. Basically, the configuration is done in server.xml file, defining maxPostSize attribute in <Connector> element.

  In any case, please, refer to Tomcat's documentation about how to apply this configuration.

Additional Information

• Tomcat's maximum POST size parameter limit:
  • https://github.com/apache/tomcat/blob/9.0.33/java/org/apache/catalina/connector/Request.java#L2897-L2930
• Liferay DXP gets full request if parameter is null:
  
  • https://github.com/liferay/liferay-dxp/blob/7.2.x/portal-impl/src/com/liferay/portal/jsonwebservice/action/JSONWebServiceInvokerAction.java#L60-L75
• Tomcat application server configuration: POST size request
  • https://tomcat.apache.org/tomcat-9.0-doc/config/http.html#Common_Attributes