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Codename One API


Networking, Storage, Filesystem & related API's

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Package Description

Networking, Storage, Filesystem & related API's

The IO package includes all of the main features related to storage and networking with the exception of SQL & XML parsing.


Storage is accessed via the Storage class. It is a flat filesystem like interface and contains the ability to list/delete and write to named storage entries.

The Storage API also provides convenient methods to write objects to Storage and read them from Storage specifically readObject & writeObject.
Notice that objects in Storage are deleted when an app is uninstalled but are retained between application updates.

The sample code below demonstrates listing the content of the storage, adding/viewing and deleting entries within the storage:

List of files within the storage Content of a file added to the storage

The Preferences API

Storage also offers a very simple API in the form of the Preferences class. The Preferences class allows developers to store simple variables, strings, numbers, booleans etc. in storage without writing any storage code. This is a common use case within applications e.g. you have a server token that you need to store you can store & read it like this:

This gets somewhat confusing with primitive numbers e.g. if you use Preferences.set("primitiveLongValue", myLongNumber) then invoke Preferences.get("primitiveLongValue", 0) you might get an exception!
This would happen because the value is physically a Long object but you are trying to get an Integer. The workaround is to remain consistent and use code like this Preferences.get("primitiveLongValue", (long)0).

File System

FileSystemStorage provides file system access. It maps to the underlying OS's file system API providing most of the common operations expected from a file API somewhat in the vain of & e.g. opening, renaming, deleting etc.

Notice that the file system API is somewhat platform specific in its behavior. All paths used the API should be absolute otherwise they are not guaranteed to work.

The main reason & weren't supported directly has a lot to do with the richness of those two API's. They effectively allow saving a file anywhere, however mobile devices are far more restrictive and don't allow apps to see/modify files that are owned by other apps.

File Paths & App Home

All paths in FileSystemStorage are absolute, this simplifies the issue of portability significantly since the concept of relativity and current working directory aren't very portable.

All URL's use the / as their path separator we try to enforce this behavior even in Windows.

Directories end with the / character and thus can be easily distinguished by their name.

The FileSystemStorage API provides a getRoots() call to list the root directories of the file system (you can then "dig in" via the listFiles API). However, this is confusing and unintuitive for developers.

To simplify the process of creating/reading files we added the getAppHomePath() method. This method allows us to obtain the path to a directory where files can be stored/read.

We can use this directory to place an image to share as we did in the share sample.

Warning: A common Android hack is to write files to the SDCard storage to share them among apps. Android 4.x disabled the ability to write to arbitrary directories on the SDCard even when the appropriate permission was requested.

A more advanced usage of the FileSystemStorage API can be a FileSystemStorage Tree:

Simple sample of a tree for the FileSystemStorage API

Storage vs. File System

The question of storage vs. file system is often confusing for novice mobile developers. This embeds two separate questions:

The main reasons for the 2 API's are technical. Many OS's provide 2 ways of accessing data specific to the app and this is reflected within the API. E.g. on Android the FileSystemStorage maps to API's such as whereas the Storage maps to Context.openFileInput().

The secondary reason for the two API's is conceptual. FileSystemStorage is more powerful and in a sense provides more ways to fail, this is compounded by the complex on-device behavior of the API. Storage is designed to be friendlier to the uninitiated and more portable.

You should pick Storage unless you have a specific requirement that prevents it. Some API's such as Capture expect a FileSystemStorage URI so in those cases this would also be a requirement.

Another case where FileSystemStorage is beneficial is the case of hierarchy or native API usage. If you need a a directory structure or need to communicate with a native API the FileSystemStorage approach is usually easier.
Warning: In some OS's the FileSystemStorage API can find the content of the Storage API. As one is implemented on top of the other. This is undocumented behavior that can change at any moment!

Network Manager & Connection Request

One of the more common problems in Network programming is spawning a new thread to handle the network operations. In Codename One this is done seamlessly and becomes unessential thanks to the NetworkManager.

NetworkManager effectively alleviates the need for managing network threads by managing the complexity of network threading. The connection request class can be used to facilitate web service requests when coupled with the JSON/XML parsing capabilities.

To open a connection one needs to use a ConnectionRequest object, which has some similarities to the networking mechanism in JavaScript but is obviously somewhat more elaborate.

You can send a get request to a URL using something like:

Notice that you can also implement the same thing and much more by avoiding the response listener code and instead overriding the methods of the ConnectionRequest class which offers multiple points to override e.g.

Notice that overriding buildRequestBody(OutputStream) will only work for POST requests and will replace writing the arguments.

Important: You don't need to close the output/input streams passed to the request methods. They are implicitly cleaned up.

NetworkManager also supports synchronous requests which work in a similar way to Dialog via the invokeAndBlock call and thus don't block the EDT illegally. E.g. you can do something like this:

Notice that in this case the addToQueueAndWait method returned after the connection completed. Also notice that this was totally legal to do on the EDT!


By default the NetworkManager launches with a single network thread. This is sufficient for very simple applications that don't do too much networking but if you need to fetch many images concurrently and perform web services in parallel this might be an issue.

Warning: Once you increase the thread count there is no guarantee of order for your requests. Requests might not execute in the order with which you added them to the queue!

To update the number of threads use:

All the callbacks in the ConnectionRequest occur on the network thread and not on the EDT!

There is one exception to this rule which is the postResponse() method designed to update the UI after the networking code completes.

Important: Never change the UI from a ConnectionRequest callback. You can either use a listener on the ConnectionRequest, use postResponse() (which is the only exception to this rule) or wrap your UI code with Display.callSerially(java.lang.Runnable).

Arguments, Headers & Methods

HTTP/S is a complex protocol that expects complex encoded data for its requests. Codename One tries to simplify and abstract most of these complexities behind common sense API's while still providing the full low level access you would expect from such an API.


HTTP supports several "request methods", most commonly GET & POST but also a few others such as HEAD, PUT, DELETE etc.

Arguments in HTTP are passed differently between GET and POST methods. That is what the setPost method in Codename One determines, whether arguments added to the request should be placed using the GET semantics or the POST semantics.

So if we continue our example from above we can do something like this:

This will implicitly add a get argument with the content of value. Notice that we don't really care what value is. It's implicitly HTTP encoded based on the get/post semantics. In this case it will use the get encoding since we passed false to the constructor.

A simpler implementation could do something like this:

This would be almost identical but doesn't provide the convenience for switching back and forth between GET/POST and it isn't as fluent.

We can skip the encoding in complex cases where server code expects illegal HTTP characters (this happens) using the addArgumentNoEncoding method. We can also add multiple arguments with the same key using addArgumentArray.


As we explained above, the setPost() method allows us to manipulate the get/post semantics of a request. This implicitly changes the POST or GET method submitted to the server.

However, if you wish to have finer grained control over the submission process e.g. for making a HEAD request you can do this with code like:


When communicating with HTTP servers we often pass data within headers mostly for authentication/authorization but also to convey various properties.

Some headers are builtin as direct API's e.g. content type is directly exposed within the API since it's a pretty common use case. We can set the content type of a post request using:

We can also add any arbitrary header type we want, e.g. a very common use case is basic authorization where the authorization header includes the Base64 encoded user/password combination as such:

This can be quite tedious to do if you want all requests from your app to use this header. For this use case you can just use:

Server Headers

Server returned headers are a bit trickier to read. We need to subclass the connection request and override the readHeaders method e.g.:

Here we can extract the headers one by one to handle complex headers such as cookies, authentication etc.

Error Handling

As you noticed above practically all of the methods in the ConectionRequest throw IOException. This allows you to avoid the try/catch semantics and just let the error propagate up the chain so it can be handled uniformly by the application.

There are two distinct placed where you can handle a networking error:

Notice that the NetworkManager error handler takes precedence thus allowing you to define a global policy for network error handling by consuming errors.

E.g. if I would like to block all network errors from showing anything to the user I could do something like this:

The error listener is invoked first with the NetworkEvent matching the error. Consuming the event prevents it from propagating further down the chain into the ConnectionRequest callbacks.

We can also override the error callbacks of the various types in the request e.g. in the case of a server error code we can do:

Important: The error callback callback is triggered in the network thread!
As a result it can't access the UI to show a Dialog or anything like that.

Another approach is to use the setFailSilently(true) method on the ConnectionRequest. This will prevent the ConnectionRequest from displaying any errors to the user. It's a very powerful strategy if you use the synchronous version of the API's e.g.:

This code will only work with the synchronous "AndWait" version of the method since the response code will take a while to return for the non-wait version.

Error Stream

When we get an error code that isn't 200/300 we ignore the result. This is problematic as the result might contain information we need. E.g. many webservices provide further XML/JSON based details describing the reason for the error code.

Calling setReadResponseForErrors(true) will trigger a mode where even errors will receive the readResponse callback with the error stream. This also means that API's like getData and the listener API's will also work correctly in case of error.


Gzip is a very common compression format based on the lz algorithm, it's used by web servers around the world to compress data.

Codename One supports GZIPInputStream and GZIPOutputStream, which allow you to compress data seamlessly into a stream and extract compressed data from a stream. This is very useful and can be applied to every arbitrary stream.

Codename One also features a GZConnectionRequest, which will automatically unzip an HTTP response if it is indeed gzipped. Notice that some devices (iOS) always request gzip'ed data and always decompress it for us, however in the case of iOS it doesn't remove the gziped header. The GZConnectionRequest is aware of such behaviors so its better to use that when connecting to the network (if applicable).

By default GZConnectionRequest doesn't request gzipped data (only unzips it when its received) but its pretty easy to do so just add the HTTP header Accept-Encoding: gzip e.g.:

Do the rest as usual and you should have smaller responses from the servers.

File Upload

MultipartRequest tries to simplify the process of uploading a file from the local device to a remote server.

You can always submit data in the buildRequestBody but this is flaky and has some limitations in terms of devices/size allowed. HTTP standardized file upload capabilities thru the multipart request protocol, this is implemented by countless servers and is well documented. Codename One supports this out of the box.

Since we assume most developers reading this will be familiar with Java here is the way to implement the multipart upload in the servlet API.

MultipartRequest is a ConnectionRequest most stuff you expect from there should work. Even addArgument etc.


Codename One has several built in parsers for JSON, XML, CSV & Properties formats. You can use those parsers to read data from the Internet or data that is shipping with your product. E.g. use the CSV data to setup default values for your application.

All our parsers are designed with simplicity and small distribution size; they don't validate and will fail in odd ways when faced with broken data. The main logic behind this is that validation takes up CPU time on the device where CPU is a precious resource.

Parsing CSV

CSV is probably the easiest to use, the "Comma Separated Values" format is just a list of values separated by commas (or some other character) with new lines to indicate another row in the table. These usually map well to an Excel spreadsheet or database table and are supported by default in all spreadsheets.

To parse a CSV just use the CSVParser class as such:

CSV parsing results, notice the properly escaped parentheses and comma

The data contains a two dimensional array of the CSV content. You can change the delimiter character by using the CSVParser constructor that accepts a character.

IMPORTANT: Notice that we used CharArrayReader for this sample. Normally you would want to use java.util.InputStreamReader for real world data.


The JSON ("Java Script Object Notation") format is popular on the web for passing values to/from webservices since it works so well with JavaScript. Parsing JSON is just as easy but has two different variations. You can use the JSONParser class to build a tree of the JSON data as such:

The response is a Map containing a nested hierarchy of Collection (java.util.List), Strings and numbers to represent the content of the submitted JSON. To extract the data from a specific path just iterate the Map keys and recurs into it.

The sample below uses results from an API of ice and fire that queries structured data about the "Song Of Ice & Fire" book series. Here is a sample result returned from the API for the query

We will place that into a file named "anapioficeandfire.json" in the src directory to make the next sample simpler:

  1. The JSONParser returns a Map which is great if the root object is a Map but in some cases its a list of elements (as is the case above). In this case a special case "root" element is created to contain the actual list of elements.
  2. We rely that the entries are all maps, this might not be the case for every API type.
  3. Notice that the "titles" and "aliases" entries are both lists of elements. We use java.util.List to avoid a clash with com.codename1.ui.List.
Parsed JSON result, clicking the elements opens the URL from the JSON

Tip: The structure of the returned map is sometimes unintuitive when looking at the raw JSON. The easiest thing to do is set a breakpoint on the method and use the inspect variables capability of your IDE to inspect the returned element hierarchy while writing the code to extract that data

An alternative approach is to use the static data parse() method of the JSONParser class and implement a callback parser e.g.: JSONParser.parse(reader, callback);

Notice that a static version of the method is used! The callback object is an instance of the JSONParseCallback interface, which includes multiple methods. These methods are invoked by the parser to indicate internal parser states, this is similar to the way traditional XML SAX event parsers work.

XML Parsing

The XMLParser started its life as an HTML parser built for displaying mobile HTML. That usage has since been deprecated but the parser can still parse many HTML pages and is very "loose" in terms of verification. This is both good and bad as the parser will work with invalid data without complaining.

The simplest usage of XMLParser looks a bit like this:

The Element contains children and attributes. It represents a tag within the XML document and even the root document itself. You can iterate over the XML tree to extract the data from within the XML file.

We have a great sample of working with XMLParser in the Tree class.

XPath Processing

Codename One ships with support to a subset of XPath processing, you can read more about it in the processing package docs.

Externalizable Objects

Codename One provides the Externalizable interface, which is similar to the Java SE Externalizable interface. This interface allows an object to declare itself as Externalizable for serialization (so an object can be stored in a file/storage or sent over the network). However, due to the lack of reflection and use of obfuscation these objects must be registered with the Util class.

Codename One doesn't support the Java SE Serialization API due to the size issues and complexities related to obfuscation.

The major objects that are supported by default in the Codename One Externalizable are: String, Collection, Map, ArrayList, HashMap, Vector, Hashtable, Integer, Double, Float, Byte, Short, Long, Character, Boolean, Object[], byte[], int[], float[], long[], double[].

Externalizing an object such as h below should work just fine:

However, notice that some things aren't polymorphic e.g. if we will externalize a String[] we will get back an Object[] since String arrays aren't detected by the implementation.

Important: The externalization process caches objects so the app will seem to work and only fail on restart!

Implementing the Externalizable interface is only important when we want to store a proprietary object. In this case we must register the object with the class so the externalization algorithm will be able to recognize it by name by invoking:

You should do this early on in the app e.g. in the init(Object) but you shouldn't do it in a static initializer within the object as that might never be invoked!

An Externalizable object must have a default public constructor and must implement the following 4 methods:

The getVersion() method returns the current version of the object allowing the stored data to change its structure in the future (the version is then passed when internalizing the object). The object id is a String uniquely representing the object; it usually corresponds to the class name (in the example above the Unique Name should be MyClass).

Warning: It's a common mistake to use getClass().getName() to implement getObjectId() and it would seem to work in the simulator. This isn't the case though!
Since devices obfuscate the class names this becomes a problem as data is stored in a random name that changes with every release.

Developers need to write the data of the object in the externalize method using the methods in the data output stream and read the data of the object in the internalize method e.g.:

Since strings might be null sometimes we also included convenience methods to implement such externalization. This effectively writes a boolean before writing the UTF to indicate whether the string is null:

Assuming we added a new date field to the object we can do the following. Notice that a Date is really a long value in Java that can be null. For completeness the full class is presented below:

Notice that we only need to check for compatibility during the reading process as the writing process always writes the latest version of the data.

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