StubFtpServer - Getting Started

StubFtpServer is a "stub" implementation of an FTP server. It supports the main FTP commands by implementing command handlers for each of the corresponding low-level FTP server commands (e.g. RETR, DELE, LIST). These CommandHandlers can be individually configured to return custom data or reply codes, allowing simulation of a complete range of both success and failure scenarios. The CommandHandlers can also be interrogated to verify command invocation data such as command parameters and timestamps.

StubFtpServer works out of the box with reasonable defaults, but can be fully configured programmatically or within a Spring Framework (or similar) container.

Here is how to start the StubFtpServer with the default configuration. This will return success reply codes, and return empty data (for retrieved files, directory listings, etc.).

StubFtpServer stubFtpServer = new StubFtpServer();

If you are running on a system where the default port (21) is already in use or cannot be bound from a user process (such as Unix), you will need to use a different server control port. Use the StubFtpServer.setServerControlPort(int serverControlPort) method to use a different port number. If you specify a value of 0, then the server will use a free port number. Then call getServerControlPort() AFTER calling start() has been called to determine the actual port number being used. Or, you can pass in a specific port number, such as 9187.


CommandHandlers are the heart of the StubFtpServer.

StubFtpServer creates an appropriate default CommandHandler for each (supported) FTP server command. See the list of CommandHandler classes associated with FTP server commands in FTP Commands and CommandHandlers.

You can retrieve the existing CommandHandler defined for an FTP server command by calling the StubFtpServer.getCommandHandler(String name) method, passing in the FTP server command name. For example:

PwdCommandHandler pwdCommandHandler = (PwdCommandHandler) stubFtpServer.getCommandHandler("PWD");

You can replace the existing CommandHandler defined for an FTP server command by calling the StubFtpServer.setCommandHandler(String name, CommandHandler commandHandler) method, passing in the FTP server command name, such as "STOR" or "USER", and the CommandHandler instance. For example:

PwdCommandHandler pwdCommandHandler = new PwdCommandHandler();
stubFtpServer.setCommandHandler("PWD", pwdCommandHandler);

Generic CommandHandlers

StubFtpServer includes a couple generic CommandHandler classes that can be used to replace the default command handler for an FTP command. See the Javadoc for more information.

  • StaticReplyCommadHandler

    StaticReplyCommadHandler is a CommandHandler that always sends back the configured reply code and text. This can be a useful replacement for a default CommandHandler if you want a certain FTP command to always send back an error reply code.

  • SimpleCompositeCommandHandler

    SimpleCompositeCommandHandler is a composite CommandHandler that manages an internal ordered list of CommandHandlers to which it delegates. Starting with the first CommandHandler in the list, each invocation of this composite handler will invoke (delegate to) the current internal CommandHander. Then it moves on the next CommandHandler in the internal list.

Configuring CommandHandler for a New (Unsupported) Command

If you want to add support for a command that is not provided out of the box by StubFtpServer, you can create a CommandHandler instance and set it within the StubFtpServer using the StubFtpServer.setCommandHandler(String name, CommandHandler commandHandler) method in the same way that you replace an existing CommandHandler (see above). The following example uses the StaticReplyCommandHandler to add support for the FEAT command.

final String FEAT_TEXT = "Extensions supported:\n" +
        "MLST size*;create;modify*;perm;media-type\n" +
        "SIZE\n" +
        "COMPRESSION\n" +
StaticReplyCommandHandler featCommandHandler = new StaticReplyCommandHandler(211, FEAT_TEXT);
stubFtpServer.setCommandHandler("FEAT", featCommandHandler);

Creating Your Own Custom CommandHandler Class

If one of the existing CommandHandler classes does not fulfill your needs, you can alternately create your own custom CommandHandler class. The only requirement is that it implement the org.mockftpserver.core.command.CommandHandler interface. You would, however, likely benefit from inheriting from one of the existing abstract CommandHandler superclasses, such as org.mockftpserver.stub.command.AbstractStubCommandHandler or org.mockftpserver.core.command.AbstractCommandHandler. See the javadoc of these classes for more information.

Retrieving Command Invocation Data

Each predefined StubFtpServer CommandHandler manages a List of InvocationRecord objects -- one for each time the CommandHandler is invoked. An InvocationRecord contains the Command that triggered the invocation (containing the command name and parameters), as well as the invocation timestamp and client host address. The InvocationRecord also contains a Map, with optional CommandHandler-specific data. See the Javadoc for more information.

You can retrieve the InvocationRecord from a CommandHandler by calling the getInvocation(int index) method, passing in the (zero-based) index of the desired invocation. You can get the number of invocations for a CommandHandler by calling numberOfInvocations(). The Example Test Using Stub Ftp Server below illustrates retrieving and interrogating an InvocationRecord from a CommandHandler.

Example Test Using StubFtpServer

This section includes a simplified example of FTP client code to be tested, and a JUnit test for it that uses StubFtpServer.

FTP Client Code

The following RemoteFile class includes a readFile() method that retrieves a remote ASCII file and returns its contents as a String. This class uses the FTPClient from the Apache Commons Net framework.

public class RemoteFile {

    private String server;

    public String readFile(String filename) throws SocketException, IOException {

        FTPClient ftpClient = new FTPClient();

        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        boolean success = ftpClient.retrieveFile(filename, outputStream);

        if (!success) {
            throw new IOException("Retrieve file failed: " + filename);
        return outputStream.toString();
    public void setServer(String server) {
        this.server = server;
    // Other methods ...

JUnit Test For FTP Client Code Using StubFtpServer

The following RemoteFileTest class includes a couple of JUnit tests that use StubFtpServer. The test illustrates replacing the default CommandHandler with a customized handler.

import org.mockftpserver.core.command.InvocationRecord;
import org.mockftpserver.stub.StubFtpServer;
import org.mockftpserver.stub.command.RetrCommandHandler;
import org.mockftpserver.test.AbstractTest;

public class RemoteFileTest extends AbstractTest {

    private static final String FILENAME = "dir/sample.txt";

    private RemoteFile remoteFile;
    private StubFtpServer stubFtpServer;
    public void testReadFile() throws Exception {

        final String CONTENTS = "abcdef 1234567890";

        // Replace the default RETR CommandHandler; customize returned file contents
        RetrCommandHandler retrCommandHandler = new RetrCommandHandler();
        stubFtpServer.setCommandHandler("RETR", retrCommandHandler);
        String contents = remoteFile.readFile(FILENAME);

        // Verify returned file contents
        assertEquals("contents", CONTENTS, contents);
        // Verify the submitted filename
        InvocationRecord invocationRecord = retrCommandHandler.getInvocation(0);
        String filename = invocationRecord.getString(RetrCommandHandler.PATHNAME_KEY);
        assertEquals("filename", FILENAME, filename);

     * Test the readFile() method when the FTP transfer fails (returns a non-success reply code) 
    public void testReadFileThrowsException() {

        // Replace the default RETR CommandHandler; return failure reply code
        RetrCommandHandler retrCommandHandler = new RetrCommandHandler();
        stubFtpServer.setCommandHandler("RETR", retrCommandHandler);

        try {
            fail("Expected IOException");
        catch (IOException expected) {
            // Expected this
    protected void setUp() throws Exception {
        remoteFile = new RemoteFile();
        stubFtpServer = new StubFtpServer();

    protected void tearDown() throws Exception {

Things to note about the above test:

  • The StubFtpServer instance is created in the setUp() method, but is not started there because it must be configured differently for each test. The StubFtpServer instance is stopped in the tearDown() method, to ensure that it is stopped, even if the test fails.

Spring Configuration

You can easily configure a StubFtpServer instance in the Spring Framework. The following example shows a Spring configuration file.

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns=""

  <bean id="stubFtpServer" class="org.mockftpserver.stub.StubFtpServer">

    <property name="commandHandlers">
        <entry key="LIST">
          <bean class="org.mockftpserver.stub.command.ListCommandHandler">
            <property name="directoryListing">
                11-09-01 12:30PM  406348 File2350.log
                11-01-01 1:30PM &lt;DIR&gt; 0 archive

        <entry key="PWD">
          <bean class="org.mockftpserver.stub.command.PwdCommandHandler">
            <property name="directory" value="foo/bar" />

        <entry key="DELE">
          <bean class="org.mockftpserver.stub.command.DeleCommandHandler">
            <property name="replyCode" value="450" />

        <entry key="RETR">
          <bean class="org.mockftpserver.stub.command.RetrCommandHandler">
            <property name="fileContents"
              value="Sample file contents line 1&#10;Line 2&#10;Line 3"/>



This example overrides the default handlers for the following FTP commands:

  • LIST - replies with a predefined directory listing
  • PWD - replies with a predefined directory pathname
  • DELE - replies with an error reply code (450)
  • RETR - replies with predefined contents for a retrieved file

And here is the Java code to load the above Spring configuration file and start the configured StubFtpServer.

ApplicationContext context = new ClassPathXmlApplicationContext("stubftpserver-beans.xml");
stubFtpServer = (StubFtpServer) context.getBean("stubFtpServer");

FTP Command Reply Text ResourceBundle

The default text asociated with each FTP command reply code is contained within the "" ResourceBundle file. You can customize these messages by providing a locale-specific ResourceBundle file on the CLASSPATH, according to the normal lookup rules of the ResourceBundle class (e.g., ""). Alternatively, you can completely replace the ResourceBundle file by calling the calling the StubFtpServer.setReplyTextBaseName(String) method.