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A Java FTP client built with Apache Commons Net 3.13.0 can connect to an FTP server, authenticate, list directories, upload and download files, and delete or rename remote files. Use FTPClient for plain FTP, FTPSClient for FTP over TLS, and a separate SSH library when the server requires SFTP.
This guide uses a defensive baseline: validate the connection reply, configure timeouts, enter local passive mode after connecting, select binary transfer mode explicitly, check Boolean operation results, complete streamed transfers, and always disconnect in cleanup.
Key takeaways
- Apache Commons Net 3.13.0 is the current verified release as of August 18, 2026, and it requires Java 8 or later.
FTPClientimplements FTP, whileFTPSClientimplements FTP over TLS; Commons Net’s FTP package does not implement SFTP.- For ordinary client-to-server transfers, enter local passive mode and set
FTP.BINARY_FILE_TYPEafter connecting. storeFileandretrieveFilereturnboolean, so failed operations require inspection ofgetReplyCode()andgetReplyString().- Stream-based methods such as
storeFileStreamandretrieveFileStreamrequirecompletePendingCommand()after the data stream closes. - Production workflows should upload under a temporary name and rename the completed file rather than exposing a partially uploaded final filename.
What is Apache Commons Net used for in Java FTP clients?
Apache Commons Net is an Apache-licensed Java networking library that provides protocol-level clients for FTP and FTPS as well as SMTP, POP3, IMAP, Telnet, NNTP, NTP, and other protocols. This article focuses on its FTP package and the classes needed for file-transfer integrations.
The central FTPClient class manages FTP’s control connection and data connections. The class exposes protocol operations and configuration rather than a complete synchronization, scheduling, monitoring, or guaranteed-delivery platform. That low-level control works well for vendor feeds, scheduled batch jobs, internal transfers, and custom application workflows, provided the application supplies lifecycle management, retries, integrity checks, and operational logging.
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The official Apache Commons Net project overview describes the broader protocol scope. The FTP-specific API is documented in the FTPClient API reference.
| Class | Purpose |
|---|---|
FTPClient |
Plain FTP connections and file-transfer commands. |
FTPSClient |
FTP with TLS, using explicit or implicit TLS modes. |
FTPFile |
Parsed metadata for a remote file or directory. |
FTPClientConfig |
Server-specific listing parser, locale, date, and format configuration. |
FTPReply |
Reply-code constants and helpers such as positive-completion checks. |
FTP |
FTP constants, including binary and ASCII file types. |
FTPConnectionClosedException |
A more specific exception for a server-side connection close, commonly associated with reply code 421. |
How do you install Apache Commons Net?
Use Apache Commons Net version 3.13.0 in Maven or Gradle. The release was published on March 15, 2026, and the project requires Java 8 or later according to the official download page and dependency information.
Maven
<dependency>
<groupId>commons-net</groupId>
<artifactId>commons-net</artifactId>
<version>3.13.0</version>
</dependency>Gradle
implementation("commons-net:commons-net:3.13.0")Commons Net is distributed under the Apache License 2.0. Ordinary FTP client use normally does not require adding Commons IO manually because the current project dependency information declares Commons IO as a compile dependency. Recheck the official release history before pinning a new application release, because 3.13.0 is the latest version verified for this article rather than a permanent promise about future releases.
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An FTP session has a control connection for commands and replies and separate data connections for listings and file contents. A reliable client constructs the object, configures timeouts, connects, validates the server reply, authenticates, selects passive mode and transfer type, performs operations, logs out, and disconnects even when an exception occurs.
Connecting alone does not prove that the server accepted the session. The FTPClient API documentation requires a connection before normal FTP operations and provides reply accessors for validating the result.
A safe baseline connection
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPReply;
import java.io.IOException;
public final class FtpConnectionExample {
public static void main(String[] args) {
String host = "ftp.example.com";
int port = 21;
String username = System.getenv("FTP_USERNAME");
String password = System.getenv("FTP_PASSWORD");
FTPClient ftp = new FTPClient();
try {
ftp.setConnectTimeout(10_000);
ftp.setDefaultTimeout(10_000);
ftp.setDataTimeout(30_000);
ftp.connect(host, port);
int reply = ftp.getReplyCode();
if (!FTPReply.isPositiveCompletion(reply)) {
throw new IOException("FTP server rejected connection: "
+ ftp.getReplyString());
}
if (!ftp.login(username, password)) {
throw new IOException("FTP login failed: "
+ ftp.getReplyString());
}
// Set these after connect: connect resets transfer-related state.
ftp.enterLocalPassiveMode();
ftp.setFileType(FTP.BINARY_FILE_TYPE);
System.out.println("Connected to " + ftp.getSystemName());
if (!ftp.logout()) {
throw new IOException("FTP logout failed: "
+ ftp.getReplyString());
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if (ftp.isConnected()) {
try {
ftp.disconnect();
} catch (IOException ignored) {
// Log this in a real application.
}
}
}
}
}Do not hard-code production passwords in source code. Inject credentials from environment variables, a secret manager, or application configuration with appropriate access controls. The example uses placeholders and environment variables for that reason.
FTPClient is not normally used as an AutoCloseable resource, so Java try-with-resources is not the ordinary cleanup pattern. Explicit logout() is useful after successful work, but disconnect() belongs in guaranteed cleanup because logout itself can fail or an earlier operation can throw.
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How do you upload a file with FTPClient?
Use storeFile(remotePath, inputStream) for a straightforward upload, close the input stream in the caller, and check the returned Boolean. The FTPClient API does not close an input stream supplied to storeFile.
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
public static void upload(FTPClient ftp, Path localFile, String remotePath)
throws IOException {
ftp.setFileType(FTP.BINARY_FILE_TYPE);
try (InputStream input = Files.newInputStream(localFile)) {
boolean uploaded = ftp.storeFile(remotePath, input);
if (!uploaded) {
throw new IOException("Upload failed: " + ftp.getReplyString());
}
}
}Binary mode is the safe default for archives, images, PDFs, executables, and most application data. Use ASCII mode only when the receiving workflow explicitly expects NETASCII text conversion. Commons Net documents ASCII as the default file type, and a connect method resets the file type to ASCII, so set binary mode after connecting even if code appears to set it earlier.
How do you upload large files or report progress?
Use storeFileStream when the application needs to copy the stream itself, measure progress, or integrate with a custom stream pipeline. A streamed FTP command is not complete merely because the data stream has been closed.
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try (InputStream input = Files.newInputStream(localFile);
OutputStream output = ftp.storeFileStream(remotePath)) {
if (output == null) {
throw new IOException("Could not open remote data stream: "
+ ftp.getReplyString());
}
input.transferTo(output);
}
if (!ftp.completePendingCommand()) {
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How do you prevent consumers from reading a partial upload?
Upload to a temporary remote name, validate the transfer, and rename the temporary file to its final name only after completion. Remote rename is not a universal transaction across servers, but it is a practical handoff convention when the server supports it.
String temporaryRemotePath = "/incoming/orders.csv.part";
String finalRemotePath = "/incoming/orders.csv";
upload(ftp, localFile, temporaryRemotePath);
// Add a size or checksum verification step when the server supports it.
if (!ftp.rename(temporaryRemotePath, finalRemotePath)) {
throw new IOException("Remote rename failed: " + ftp.getReplyString());
}Also define what should happen after a timeout: the application may need to inspect, remove, or resume the temporary file. Duplicate delivery, overwriting, and replay behavior should be explicit in a batch workflow.
How do you download a file with FTPClient?
Use retrieveFile(remotePath, outputStream) for a complete download and check its Boolean result before treating the local file as valid.
import org.apache.commons.net.ftp.FTP;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Path;
public static void download(FTPClient ftp, String remotePath, Path localFile)
throws IOException {
ftp.setFileType(FTP.BINARY_FILE_TYPE);
try (OutputStream output = Files.newOutputStream(localFile)) {
boolean downloaded = ftp.retrieveFile(remotePath, output);
if (!downloaded) {
throw new IOException("Download failed: " + ftp.getReplyString());
}
}
}For safer processing, download to a local temporary path, verify the size or content, and move the file into the application’s processing directory only after validation. Downloaded files are untrusted input: validate their size, expected type, and content before parsing or executing anything.
How do you stream a download?
Use retrieveFileStream when the application needs direct control over the data stream, and call completePendingCommand() after both streams close.
try (InputStream input = ftp.retrieveFileStream(remotePath);
OutputStream output = Files.newOutputStream(localFile)) {
if (input == null) {
throw new IOException("Could not open remote data stream: "
+ ftp.getReplyString());
}
input.transferTo(output);
}
if (!ftp.completePendingCommand()) {
throw new IOException("FTP server did not complete download: "
+ ftp.getReplyString());
}If the stream method returns null, no usable data stream was opened. If the final completion call returns false, the server rejected or failed the command after the preliminary data-channel response. Both cases require the reply code and reply text in logs.
How do you list FTP files and navigate directories?
Use listFiles(path) when you need parsed metadata and listNames(path) when you need only names. Commons Net represents parsed entries as FTPFile objects.
import org.apache.commons.net.ftp.FTPFile;
FTPFile[] files = ftp.listFiles("/incoming");
for (FTPFile file : files) {
System.out.printf("%s %s %d%n",
file.isDirectory() ? "DIR " : "FILE",
file.getName(),
file.getSize());
}
String[] names = ftp.listNames("/incoming");
System.out.println("Names returned: " + (names == null ? 0 : names.length));Common directory and metadata operations include:
String current = ftp.printWorkingDirectory();
boolean changed = ftp.changeWorkingDirectory("/incoming");
boolean parent = ftp.changeToParentDirectory();
boolean made = ftp.makeDirectory("/archive");
boolean removed = ftp.removeDirectory("/empty-directory");
boolean deleted = ftp.deleteFile("/incoming/file.txt");
String modificationTime = ftp.getModificationTime("/incoming/file.txt");Where the server supports the relevant command, mdtmFile(path) can provide modification-time information. Check every Boolean result and do not assume that a server account has permission to create, delete, rename, or navigate every path.
Why can FTP directory listings fail even when login works?
FTP directory listings use a data connection and server-specific output formats, so successful authentication does not guarantee that listFiles will work. Server operating system, locale, date format, command support, and nonstandard formatting can all affect parsing.
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Commons Net provides parser support and FTPClientConfig for listing configuration. A localized or unusual LIST response may still require explicit configuration or a custom parser. The 3.13.0 release notes specifically record a fix involving Linux vsftpd listings in Chinese or Japanese locales.
Prefer machine-oriented listing commands such as MLSD or MLST when the server supports them and the application needs stable metadata. Do not treat all server listings as interchangeable, and test against the actual vendor endpoint rather than only a local FTP server.
What is the difference between passive and active FTP?
Active FTP asks the server to connect back to the client for the data connection, while passive FTP asks the client to connect to a server-advertised data port. Passive mode is generally easier for clients behind NAT devices and firewalls, but it requires the server’s passive port range, advertised address, and firewall rules to be correct.
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| Mode | Data connection direction | Typical client-side concern |
|---|---|---|
| Active | Server connects back to the client. | Inbound firewall and NAT rules commonly block the callback. |
| Local passive | Client connects to the server’s advertised data port. | Server passive range or NAT address may be misconfigured. |
| Remote passive/active | Used for server-to-server transfers. | Do not substitute these for ordinary client-to-server passive mode. |
enterRemotePassiveMode() and enterRemoteActiveMode() are for server-to-server FTP operations. Ordinary applications downloading from or uploading to a server should normally use local passive mode.
How do you troubleshoot a bad PASV address?
If a passive transfer attempts to connect to a private or unroutable address, the server or its NAT device may be returning an unusable address in the PASV response. For IPv4, setUseEPSVwithIPv4(true) can allow EPSV to succeed by using only the port when the server’s PASV address is wrong.
ftp.setUseEPSVwithIPv4(true);
ftp.enterLocalPassiveMode();The Commons Net FTPClient documentation also exposes passive-address and NAT-workaround settings. Use those settings deliberately: blindly trusting or replacing a server-supplied address can create a connection to the wrong host or conceal a server-side configuration error. The durable fix is usually to configure the server’s passive address and port range correctly and permit that range through the firewall.
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Use FTPSClient when the provider offers FTP over TLS. Explicit FTPS commonly starts on the standard FTP control port and upgrades the connection with TLS; implicit FTPS is commonly associated with port 990. The provider’s documented endpoint and TLS mode are authoritative.
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPSClient;
import java.io.IOException;
FTPSClient ftps = new FTPSClient(false); // explicit TLS
try {
ftps.setConnectTimeout(10_000);
ftps.setDefaultTimeout(10_000);
ftps.setDataTimeout(30_000);
ftps.connect(host, 21);
if (!ftps.login(username, password)) {
throw new IOException("FTPS login failed: " + ftps.getReplyString());
}
ftps.execPBSZ(0);
ftps.execPROT("P");
ftps.enterLocalPassiveMode();
ftps.setFileType(FTP.BINARY_FILE_TYPE);
// Transfer files here.
} finally {
if (ftps.isConnected()) {
ftps.disconnect();
}
}Protecting the control connection is not enough if the data connection remains clear. PBSZ and PROT must match the server’s requirements; PROT P requests private data-channel protection.
FTPS is not automatically secure merely because the class is named FTPSClient. The FTPSClient API documentation warns that hostname verification is not enabled by default and exposes hostname-verifier and endpoint-checking controls. Production code should use a properly configured trust store and enable strict certificate and hostname validation. Do not use trust-all certificates or permissive hostname verifiers outside an isolated test environment.
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What is the difference between FTP, FTPS, and SFTP?
FTP, FTPS, and SFTP are different protocols with different security models and client implementations. Commons Net directly supports FTP and FTPS, not SFTP.
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|---|---|---|---|
| FTP | No encryption for credentials or data. | FTPClient |
The endpoint is explicitly plain FTP and the network and data policy permit it. |
| FTPS | FTP with TLS for the control and, when configured, data channels. | FTPSClient |
An existing FTP service requires TLS. |
| SFTP | SSH-based file transfer. | Not provided by Commons Net’s FTP package. | The provider supplies an SSH host, key, or SFTP endpoint. |
If the provider gives an ftp:// endpoint, use FTPClient. If the provider gives FTP-with-TLS instructions, use FTPSClient. If the provider says SFTP or supplies SSH credentials, use an SSH-based SFTP library instead of adapting FTPClient. SFTP is not simply “secure FTP.”
How should you configure timeouts and keep-alives?
Configure connection, control-channel, and data-channel timeouts separately because a server can accept a connection quickly while a listing or large transfer stalls later.
ftp.setConnectTimeout(10_000); // Time allowed to establish the socket
ftp.setDefaultTimeout(10_000); // Default control-channel timeout
ftp.setDataTimeout(30_000); // Data-channel connect/read timeout| Setting | What it limits | Typical symptom when too short |
|---|---|---|
| Connect timeout | Establishing the control socket. | Slow or distant server appears unreachable. |
| Default timeout | Waiting for control-channel responses. | Login or command appears to hang. |
| Data timeout | Opening or reading a listing or transfer data connection. | Listing or file transfer fails while login works. |
| Application job timeout | The entire business operation. | A stuck transfer consumes a scheduler or worker indefinitely. |
For long transfers or control connections that remain idle, Commons Net provides setControlKeepAliveTimeout and setControlKeepAliveReplyTimeout. These settings can help with routers or servers that mishandle idle connections, as noted in the Commons Net release history. Keep-alives do not repair a blocked data port, and timeout values should reflect the slowest expected server rather than an ideal local network.
How do you handle FTP reply codes and failed operations?
Inspect both the numeric reply code and text whenever an operation fails. Commons Net methods often return false instead of throwing an exception for an FTP-level rejection.
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int code = ftp.getReplyCode();
String text = ftp.getReplyString();
if (!ftp.deleteFile("/incoming/file.txt")) {
throw new IOException("Delete failed; FTP reply "
+ code + ": " + text);
}Separate Java-side transport failures from server replies. An IOException can indicate a socket, timeout, or stream problem. A failed login can indicate bad credentials, account restrictions, or an authentication-policy mismatch. A failed file operation can indicate a missing path, permissions, quota, name collision, or a data-channel failure.
| Symptom | Likely cause | Diagnostic or recovery |
|---|---|---|
Login returns false |
Bad credentials or account restriction. | Log the reply code and message without logging the password; verify server policy. |
| Connect succeeds but listing hangs | Data channel blocked. | Use local passive mode; check the server passive range and firewall. |
| Passive transfer targets a private IP | Broken NAT or PASV configuration. | Try EPSV where appropriate and correct server NAT configuration. |
Upload or download returns false |
Permission, path, quota, or server transfer error. | Inspect getReplyCode() and getReplyString(). |
| First streamed transfer works but the second fails | Missing completePendingCommand(). |
Call it after closing the data stream. |
| Server disconnects during idle time | Server or intermediary timeout, often reply 421. |
Use keep-alives where suitable or reconnect at a safe retry boundary. |
| FTPS handshake fails | Trust-store, protocol, or hostname mismatch. | Use a valid trust store and strict hostname validation; inspect TLS diagnostics. |
| FTPS login works but transfer fails | Data-channel protection mismatch. | Configure PBSZ and PROT according to the server. |
A preliminary positive response does not replace the final completion response. This is why streamed methods need an explicit completion call and why the reply should be recorded at the point of failure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you handle encoding and non-ASCII FTP filenames?
FTP control-channel encoding and directory-listing parsing affect filenames containing non-ASCII characters. Enable UTF-8 autodetection only when the server correctly advertises or supports UTF-8 rather than assuming that every FTP server does.
ftp.setAutodetectUTF8(true);When filenames are garbled or listings cannot be parsed, verify the server’s advertised encoding, the client control encoding, and the listing parser configuration. Use FTPClientConfig for server-specific locale, date, and listing-format settings. A custom parser may be necessary for a genuinely nonstandard listing. Test names containing the actual languages and characters used by the vendor.
How do resume and atomic transfer patterns work?
Commons Net exposes restart support, including setRestartOffset(offset), but resume behavior depends on server support and must be validated with the actual endpoint. A restart offset does not by itself prove that the remote file and local file have matching content up to that point.
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For reliable batch delivery, use this sequence:
- Upload to a temporary remote filename.
- Complete the transfer and check the final FTP reply.
- Verify size or checksum when the server supports a suitable command.
- Rename the temporary file to the final name.
- Make downstream consumers ignore temporary suffixes such as
.part.
Retries should be designed around idempotency. A retry after an uncertain timeout may create a duplicate or overwrite an existing file, so choose naming, overwrite, resume, and cleanup rules deliberately.
What security practices should a production FTP client follow?
- Prefer FTPS or SFTP over plain FTP when credentials or file contents must be protected in transit.
- Never hard-code production credentials; use a secret manager, environment variables, or injected configuration.
- Validate FTPS certificate chains and hostnames with a controlled trust store.
- Use least-privilege server accounts restricted to the required directories and commands.
- Restrict application paths to the intended remote directory and guard against unintended overwrites.
- Do not log passwords, secrets, or sensitive filenames.
- Set connection, control, data, and overall job timeouts.
- Treat downloaded files as untrusted input and validate size, type, and content before processing.
- Use temporary names and final renames for files consumed by another process.
- Define duplicate-delivery, replay, timeout, and partial-file cleanup behavior.
Plain FTP sends credentials and data without encryption. Port 21 normally identifies standard FTP or explicit FTPS negotiation; implicit FTPS is commonly associated with port 990, but the server’s configuration is authoritative. A port number alone is not proof that a session is encrypted.
When should you choose SFTP, an integration framework, or managed MFT?
Apache Commons Net is a good fit when the application needs direct FTP or FTPS access, custom protocol control, and a mature Apache-licensed dependency. The application team must still implement lifecycle handling, retries, logging, integrity checks, and workflow semantics.
Choose an SSH-based SFTP library when the endpoint is SFTP. Choose Apache Camel or a similar integration framework when the requirement includes scheduled routes, polling, retries, moves, monitoring, and broader enterprise integration patterns. Choose a managed file-transfer platform when centralized auditing, partner onboarding, key management, alerting, policy enforcement, or regulated delivery matters more than embedding a low-level client. Frameworks and platforms add operational capabilities but also add configuration, infrastructure, and sometimes licensing complexity.
| Requirement | Appropriate direction | Trade-off |
|---|---|---|
| Custom FTP or FTPS commands inside a Java service | Apache Commons Net | More application code for retries, monitoring, and integrity. |
| SSH-based endpoint | SFTP-capable SSH library | Different protocol and authentication model. |
| Scheduled polling and routed transfers | Integration framework | More abstraction and configuration. |
| Central governance and partner operations | Managed MFT platform | Additional infrastructure or licensing and less low-level control. |
Production checklist
- Pin and periodically review the Commons Net version; 3.13.0 is the latest version verified for this article on August 18, 2026.
- Confirm whether the endpoint is FTP, explicit FTPS, implicit FTPS, or SFTP.
- Validate the connection reply before logging in.
- Configure connect, control, data, and job-level timeouts.
- Enter local passive mode after connecting.
- Set binary mode after connecting unless the workflow explicitly requires ASCII.
- Check every Boolean operation result and include reply code and text in safe diagnostics.
- Call
completePendingCommand()after every stream-based upload or download. - Test passive port ranges, NAT addresses, TLS certificates, locales, and representative filenames against the real server.
- Use temporary remote names, verification, and final rename for downstream handoffs.
- Disconnect in guaranteed cleanup and define safe retry and reconnection boundaries.
Frequently Asked Questions
Does Apache Commons Net support SFTP?
No. Apache Commons Net provides FTP through FTPClient and FTP over TLS through FTPSClient; SFTP is an SSH-based protocol and requires a separate SFTP-capable SSH library.
Why must passive mode be enabled after connect()?
Calling a Commons Net connect method resets the FTP data mode to active. Call enterLocalPassiveMode() after connecting so ordinary client-to-server listings and transfers use passive mode.
Why is completePendingCommand() necessary?
Stream-based methods such as retrieveFileStream and storeFileStream leave the FTP command transaction pending after the data stream closes. Calling completePendingCommand() reads the final server response and keeps the control connection synchronized.
Is FTPS secure by default in Apache Commons Net?
FTPS encrypts FTP with TLS, but production security still requires a valid trust configuration and hostname verification. The FTPSClient API documents that hostname verification is not enabled by default, so applications must configure and test it explicitly.
What file type should a Java FTP client use?
Use FTP.BINARY_FILE_TYPE for binary files and most automated application transfers. Use FTP.ASCII_FILE_TYPE only when the remote workflow specifically requires NETASCII text conversion, and set the type after connecting.
The Bottom Line
Apache Commons Net is a practical low-level Java FTP and FTPS client when the application needs direct control over file transfers. The safe baseline is to validate replies, use timeouts and passive mode, set binary mode after connecting, close streams correctly, complete pending commands, protect FTPS certificates and hostnames, and publish completed files through a temporary-name-and-rename workflow. Use an SFTP library for SFTP endpoints and a higher-level integration or managed MFT platform when the requirement extends beyond protocol operations.
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