File Handling and I/O - Working with the Java Platform - Java in a Nutshell, 6th Edition (2015)

Java in a Nutshell, 6th Edition (2015)

Part II. Working with the Java Platform

Chapter 10. File Handling and I/O

Java has had input/output (I/O) support since the very first version. However, due to Java’s strong desire for platform independence, the earlier versions of I/O functionality emphasized portability over functionality. As a result, they were not always easy to work with.

We’ll see later in the chapter how the original APIs have been supplemented—they are now rich, fully featured, and very easy to develop with. Let’s kick off the chapter by looking at the original, “classic” approach to Java I/O, which the more modern approaches layer on top of.

Classic Java I/O

The File class is the cornerstone of Java’s original way to do file I/O. This abstraction can represent both files and directories, but in doing so is sometimes a bit cumbersome to deal with, and leads to code like this:

// Get a file object to represent the user's home directory

File homedir = new File(System.getProperty("user.home"));

// Create an object to represent a config file (should

// already be present in the home directory)

File f = new File(homedir, "app.conf");

// Check the file exists, really is a file & is readable

if (f.exists() && f.isFile() && f.canRead()) {

// Create a file object for a new configuration directory

File configdir = new File(f, ".configdir");

// And create it


// Finally, move the config file to its new home

f.renameTo(new File(configdir, ".config"));


This shows some of the flexibility possible with the File class, but also demonstrates some of the problems with the abstraction. It is very general, and this requires a lot of methods to interrogate a File object in order to determine what it actually represents and its capabilities.


The File class has a very large number of methods on it, but some basic functionality (notably a way to read the contents of a file) is not, and never has been provided directly.

Here’s a quick summary of File methods:

// Permissions management

boolean canX = f.canExecute();

boolean canR = f.canRead();

boolean canW = f.canWrite();

boolean ok;

ok = f.setReadOnly();

ok = f.setExecutable(true);

ok = f.setReadable(true);

ok = f.setWritable(false);

// Different views of the file's name

File absF = f.getAbsoluteFile();

File canF = f.getCanonicalFile();

String absName = f.getAbsolutePath();

String canName = f.getCanonicalPath();

String name = f.getName();

String pName = getParent();

URI fileURI = f.toURI(); // Create URI for File path

// File metadata

boolean exists = f.exists();

boolean isAbs = f.isAbsolute();

boolean isDir = f.isDirectory();

boolean isFile = f.isFile();

boolean isHidden = f.isHidden();

long modTime = f.lastModified(); // milliseconds since epoch

boolean updateOK = f.setLastModified(updateTime); // milliseconds

long fileLen = f.length();

// File management operations

boolean renamed = f.renameTo(destFile);

boolean deleted = f.delete();

// Create won't overwrite existing file

boolean createdOK = f.createNewFile();

// Temporary file handling

File tmp = File.createTempFile("my-tmp", ".tmp");


// Directory handling

boolean createdDir = dir.mkdir();

String[] fileNames = dir.list();

File[] files = dir.listFiles();

The File class also has a few methods on it that aren’t a perfect fit for the abstraction. They largely involve interrogating the filesystem (e.g., inquiring about available free space):

long free, total, usable;

free = f.getFreeSpace();

total = f.getTotalSpace();

usable = f.getUsableSpace();

File[] roots = File.listRoots(); // all available Filesystem roots


The I/O stream abstraction (not to be confused with the streams that are used when dealing with the Java 8 Collection APIs) was present in Java 1.0, as a way of dealing with sequential streams of bytes from disks or other sources.

The core of this API is a pair of abstract classes, InputStream and OutputStream. These are very widely used, and in fact the “standard” input and output streams, which are called and System.out, are streams of this type. They are public, static fields of the System class, and are often used in even the simplest programs:

System.out.println("Hello World!");

Specific subclasses of streams, including FileInputStream, and FileOutputStream can be used to operate on individual bytes in a file—for example, by counting all the times ASCII 97 (small letter a) occurs in a file:

try (InputStream is = new FileInputStream("/Users/ben/cluster.txt")) {

byte[] buf = new byte[4096];

int len, count = 0;

while ((len = > 0) {

for (int i=0; i<len; i++)

if (buf[i] == 97) count++;


System.out.println("'a's seen: "+ count);

} catch (IOException e) {



This approach to dealing with on-disk data can lack some flexibility—most developers think in terms of characters, not bytes. To allow for this, the streams are usually combined with the higher-level Reader and Writer classes, that provide a character-stream level of interaction, rather than the low-level byte stream provided by InputStream and OutputStream and their subclasses.

Readers and Writers

By moving to an abstraction that deals in characters, rather than bytes, developers are presented with an API that is much more familiar, and that hides many of the issues with character encoding, Unicode, and so on.

The Reader and Writer classes are intended to overlay the byte stream classes, and to remove the need for low-level handling of I/O streams. They have several subclasses that are often used to layer on top of each other, such as:

§ FileReader

§ BufferedReader

§ InputStreamReader

§ FileWriter

§ PrintWriter

§ BufferedWriter

To read all lines in from a file, and print them out, we use a BufferedReader layered on top of a FileReader, like this:

try (BufferedReader in =

new BufferedReader(new FileReader(filename))) {

String line;

while((line = in.readLine()) != null) {



} catch (IOException e) {

// Handle FileNotFoundException, etc. here


If we need to read in lines from the console, rather than a file, we will usually use an InputStreamReader applied to Let’s look at an example where we want to read in lines of input from the console, but treat input lines that start with a special character as special—commands (“metas”) to be processed, rather than regular text. This is a common feature of many chat programs, including IRC. We’ll use regular expressions from Chapter 9 to help us:

Pattern SHELL_META_START = Pattern.compile("^#(\\w+)\\s*(\\w+)?");

try (BufferedReader console =

new BufferedReader(new InputStreamReader( {

String line;

READ: while((line = console.readLine()) != null) {

// Check for special commands ("metas")

Matcher m = SHELL_META_START.matcher(line);

if (m.find()) {

String metaName =;

String arg =;

doMeta(metaName, arg);

continue READ;




} catch (IOException e) {

// Handle FileNotFoundException, etc. here


To output text to a file, we can use code like this:

File f = new File(System.getProperty("user.home")

+ File.separator + ".bashrc");

try (PrintWriter out

= new PrintWriter(new BufferedWriter(new FileWriter(f)))) {

out.println("## Automatically generated config file. DO NOT EDIT");

} catch (IOException iox) {

// Handle exceptions


This older style of Java I/O has a lot of other functionality that is occasionally useful. For example, to deal with text files, the FilterInputStream class is quite often useful. Or for threads that want to communicate in a way similar to the classic “piped” I/O approach,PipedInputStream, PipedReader, and their write counterparts are provided.

Throughout this chapter so far, we have used the language feature known as "try-with-resources” (TWR). This syntax was briefly introduced in “The try-with-resources Statement”, but it is in conjunction with operations like I/O that it comes into its fullest potential, and it has granted a new lease on life to the older I/O style.

try-with-resources Revisited

To make the most of Java’s I/O capabilities, it is important to understand how and when to use TWR. It is very easy to understand when code should use TWR—whenever it is possible to do so.

Before TWR, resources had to be closed manually, and complex interactions between resources that could fail to close led to buggy code that could leak resources.

In fact, Oracle’s engineers estimate that 60% of the resource handling code in the initial JDK 6 release was incorrect. So, if even the platform authors can’t reliably get manual resource handling right, then all new code should definitely be using TWR.

The key to TWR is a new interface—AutoCloseable. This is a new interface (appears in Java 7) that is a direct superinterface of Closeable. It marks a resource that must be automatically closed, and for which the compiler will insert special exception-handling code.

Inside a TWR resource clause, only declarations of objects that implement AutoCloseable objects may appear—but the developer may declare as many as required:

try (BufferedReader in = new BufferedReader(

new FileReader("profile"));

PrintWriter out = new PrintWriter(

new BufferedWriter(

new FileWriter("profile.bak")))) {

String line;

while((line = in.readLine()) != null) {



} catch (IOException e) {

// Handle FileNotFoundException, etc. here


The consequences of this are that resources are automatically scoped to the try block. The resources (whether readable or writable) are automatically closed in the correct order, and the compiler inserts exception handling that takes dependencies between resources into account.

The overall effect of TWR is similar to C#’s using keyword, and the developer may regard it as “finalization done right.” As noted in “Finalization”, new code should never directly use the finalization mechanism, and should always use TWR instead. Older code should be refactored to use TWR as soon as is practicable.

Problems with Classic I/O

Even with the welcome addition of try-with-resources, the File class and friends have a number of problems that make them less than ideal for extensive use when performing even standard I/O operations. For instance:

§ “Missing methods” for common operations

§ Does not deal with filenames consistently across platforms

§ Fails to have a unified model for file attributes (e.g., modeling read/write access)

§ Difficult to traverse unknown directory structures

§ No platform or OS–specific features

§ Nonblocking operations for filesystems not supported

To deal with these shortcomings, Java’s I/O has evolved over several major releases. It was really with the release of Java 7 that this support became truly easy and effective to use.

Modern Java I/O

Java 7 brought in a brand new I/O API—usually called NIO.2—and it should be considered almost a complete replacement for the original File approach to I/O. The new classes are contained in the java.nio.file package.

The new API that was brought in with Java 7 is considerably easier to use for many use cases. It has two major parts. The first is a new abstraction called Path (which can be thought of as representing a file location, which may or may not have anything actually at that location). The second piece is lots of new convenience and utility methods to deal with files and filesystems. These are contained as static methods in the Files class.


For example, when using the new Files functionality, a basic copy operation is now as simple as:

File inputFile = new File("input.txt");

try (InputStream in = new FileInputStream(inputFile)) {

Files.copy(in, Paths.get("output.txt"));

} catch(IOException ex) {



Let’s take a quick survey of some of the major methods in Files—the operation of most of them is pretty self-explanatory. In many cases, the methods have return types. We have omitted handling these, as they are rarely useful except for contrived examples, and for duplicating the behavior of the equivalent C code:

Path source, target;

Attributes attr;

Charset cs = StandardCharsets.UTF_8;

// Creating files


// Example of path --> /home/ben/.profile

// Example of attributes --> rw-rw-rw-

Files.createFile(target, attr);

// Deleting files


boolean deleted = Files.deleteIfExists(target);

// Copying/Moving files

Files.copy(source, target);

Files.move(source, target);

// Utility methods to retrieve information

long size = Files.size(target);

FileTime fTime = Files.getLastModifiedTime(target);


Map<String, ?> attrs = Files.readAttributes(target, "*");


// Methods to deal with file types

boolean isDir = Files.isDirectory(target);

boolean isSym = Files.isSymbolicLink(target);

// Methods to deal with reading and writing

List<String> lines = Files.readAllLines(target, cs);

byte[] b = Files.readAllBytes(target);

BufferedReader br = Files.newBufferedReader(target, cs);

BufferedWriter bwr = Files.newBufferedWriter(target, cs);

InputStream is = Files.newInputStream(target);

OutputStream os = Files.newOutputStream(target);

Some of the methods on Files provide the opportunity to pass optional arguments, to provide additional (possibly implementation-specific) behavior for the operation.

Some of the API choices here produce occasionally annoying behavior. For example, by default, a copy operation will not overwrite an existing file, so we need to specify this behavior as a copy option:

Files.copy(Paths.get("input.txt"), Paths.get("output.txt"),


StandardCopyOption is an enum that implements an interface called CopyOption. This is also implemented by LinkOption. So Files.copy() can take any number of either LinkOption or StandardCopyOption arguments. LinkOption is used to specify how symbolic linksshould be handled (provided the underlying OS supports symlinks, of course).


Path is a type that may be used to locate a file in a filesystem. It represents a path that is:

§ System dependent

§ Hierarchical

§ Composed of a sequence of path elements

§ Hypothetical (may not exist yet, or may have been deleted)

It is therefore fundamentally different to a File. In particular, the system dependency is manifested by Path being an interface, not a class. This enables different filesystem providers to each implement the Path interface, and provide for system-specific features while retaining the overall abstraction.

The elements of a Path consist of an optional root component, which identifies the filesystem hierarchy that this instance belongs to. Note that, for example, relative Path instances may not have a root component. In addition to the root, all Path instances have zero or more directory names and a name element.

The name element is the element farthest from the root of the directory hierarchy and represents the name of the file or directory. The Path can be thought of consisting of the path elements joined together by a special separator or delimiter.

Path is an abstract concept; it isn’t necessarily bound to any physical file path. This allows us to talk easily about the locations of files that don’t exist yet. Java ships with a Paths class that provides factory methods for creating Path instances.

Paths provides two get() methods for creating Path objects. The usual version takes a String, and uses the default filesystem provider. The URI version takes advantage of the ability of NIO.2 to plug in additional providers of bespoke filesystems. This is an advanced usage, and interested developers should consult the primary documentation:

Path p = Paths.get("/Users/ben/cluster.txt");

Path p = Paths.get(new URI("file:///Users/ben/cluster.txt"));


File f = p.toFile();


Path p3 = f.toPath();


This example also shows the easy interoperation between Path and File objects. The addition of a toFile() method to Path and a toPath() method to File allows the developer to move effortlessly between the two APIs and allows for a straightforward approach to refactoring the internals of code based on File to use Path instead.

We can also make use of some useful “bridge” methods that the Files class also provides. These provide convenient access to the older I/O APIs—for example, by providing convenience methods to open Writer objects to specified Path locations:

Path logFile = Paths.get("/tmp/app.log");

try (BufferedWriter writer =

Files.newBufferedWriter(logFile, StandardCharsets.UTF_8,

StandardOpenOption.WRITE)) {

writer.write("Hello World!");

// ...

} catch (IOException e) {

// ...


We’re making use of the StandardOpenOption enum, which provides similar capabilities to the copy options, but for the case of opening a new file instead.

In this example use case, we have used the Path API to:

§ Create a Path corresponding to a new file

§ Use the Files class to create that new file

§ Open a Writer to that file

§ Write to that file

§ Automatically close it when done

In our next example, we’ll build on this to manipulate a .jar file as a FileSystem in its own right, modifying it to add an additional file directly into the JAR. JAR files are just ZIP files, so this technique will also work for .zip archives:

Path tempJar = Paths.get("sample.jar");

try (FileSystem workingFS =

FileSystems.newFileSystem(tempJar, null)) {

Path pathForFile = workingFS.getPath("/hello.txt");

List<String> ls = new ArrayList<>();

ls.add("Hello World!");

Files.write(pathForFile, ls, Charset.defaultCharset(),

StandardOpenOption.WRITE, StandardOpenOption.CREATE);


This shows how we use a FileSystem to make the Path objects inside it, via the getPath() method. This enables the developer to effectively treat FileSystem objects as black boxes.

One of the criticisms of Java’s original I/O APIs was the lack of support for native and high-performance I/O. A solution was initially added in Java 1.4, the Java New I/O (NIO) API, and it has been successively refined in successive Java versions.

NIO Channels and Buffers

NIO buffers are a low-level abstraction for high-performance I/O. They provide a container for a linear sequence of elements of a specific primitive type. We’ll work with the ByteBuffer (the most common case) in our examples.


This is a sequence of bytes, and can conceptually be thought of as a performance-critical alternative to working with a byte[]. To get the best possible performance, ByteBuffer provides support for dealing directly with the native capabilities of the platform the JVM is running on.

This approach is called the “direct buffers” case, and it bypasses the Java heap wherever possible. Direct buffers are allocated in native memory, not on the standard Java heap, and they are not subject to garbage collection in the same way as regular on-heap Java objects.

To obtain a direct ByteBuffer, call the allocateDirect() factory method. An on-heap version, allocate(), is also provided, but in practice this is not often used.

A third way to obtain a byte buffer is to wrap an existing byte[]—this will give an on-heap buffer that serves to provide a more object-oriented view of the underlying bytes:

ByteBuffer b = ByteBuffer.allocateDirect(65536);

ByteBuffer b2 = ByteBuffer.allocate(4096);

byte[] data = {1, 2, 3};

ByteBuffer b3 = ByteBuffer.wrap(data);

Byte buffers are all about low-level access to the bytes. This means that developers have to deal with the details manually—including the need to handle the endianness of the bytes and the signed nature of Java’s integral primitives:


int capacity = b.capacity();

int position = b.position();

int limit = b.limit();

int remaining = b.remaining();

boolean more = b.hasRemaining();

To get data in or out of a buffer, we have two types of operation—single value, which reads or writes a single value, and bulk, which takes a byte[] or ByteBuffer and operates on a (potentially large) number of values as a single operation. It is from the bulk operations that performance gains would expect to be realized:






double d = b.getDouble();

b.get(data, 0, data.length);

The single value form also supports a form used for absolute positioning within the buffer:

b.put(0, (byte)9);

Buffers are an in-memory abstraction. To affect the outside world (e.g., the file or network), we need to use a Channel, from the package java.nio.channels. Channels represent connections to entities that can support read or write operations. Files and sockets are the usual examples of channels, but we could consider custom implementations used for low-latency data processing.

Channels are open when they’re created, and can subsequently be closed. Once closed, they cannot be reopened. Channels are usually either readable or writable, but not both. The key to understanding channels is that:

§ Reading from a channel puts bytes into a buffer

§ Writing to a channel takes bytes from a buffer

For example, suppose we have a large file that we want to checksum in 16M chunks:

FileInputStream fis = getSomeStream();

boolean fileOK = true;

try (FileChannel fchan = fis.getChannel()) {

ByteBuffer buffy = ByteBuffer.allocateDirect(16 * 1024 * 1024);

while( != -1 || buffy.position() > 0 || fileOK) {

fileOK = computeChecksum(buffy);



} catch (IOException e) {

System.out.println("Exception in I/O");


This will use native I/O as far as possible, and will avoid a lot of copying of bytes on and off the Java heap. If the computeChecksum() method has been well implemented, then this could be a very performant implementation.

Mapped Byte Buffers

These are a type of direct byte buffer that contain a memory-mapped file (or a region of one). They are created from a FileChannel object, but note that the File object corresponding to the MappedByteBuffer must not be used after the memory-mapped operations, or an exception will be thrown. To mitigate this, we again use try-with-resources, to scope the objects tightly:

try (RandomAccessFile raf =

new RandomAccessFile(new File("input.txt"), "rw");

FileChannel fc = raf.getChannel();) {

MappedByteBuffer mbf =, 0, fc.size());

byte[] b = new byte[(int)fc.size()];

mbf.get(b, 0, b.length);

for (int i=0; i<fc.size(); i++) {

b[i] = 0; // Won't be written back to the file, we're a copy



mbf.put(b); // Zeros the file


Even with buffers, there are limitations of what can be done in Java for large (e.g., transferring 10G between filesystems) I/O operations that perform synchronously on a single thread. Before Java 7, these types of operations would typically be done by writing custom multithreaded code, and managing a separate thread for performing a background copy. Let’s move on to look at the new asynchronous I/O features that were added with JDK 7.

Async I/O

The key to the new asynchronous functionality are some new subclasses of Channel that can deal with I/O operations that need to be handed off to a background thread. The same functionality can be applied to large, long-running operations, and to several other use cases.

In this section, we’ll deal exclusively with AsynchronousFileChannel for file I/O, but there are a couple of other asynchronous channels to be aware of. We’ll deal with asynchronous sockets at the end of the chapter. We’ll look at:

§ AsynchronousFileChannel for file I/O

§ AsynchronousSocketChannel for client socket I/O

§ AsynchronousServerSocketChannel for asynchronous sockets that accept incoming connections

There are two different ways to interact with an asynchronous channel—Future style, and callback style.

Future-Based Style

We’ll meet the Future interface in detail in Chapter 11, but for the purpose of this chapter, it can be thought of as an ongoing task that may or may not have completed yet. It has two key methods:


Returns a Boolean indicating whether the task has finished.


Returns the result. If finished, returns immediately. If not finished, blocks until done.

Let’s look at an example of a program that reads a large file (possibly as large as 100 Mb) asynchronously:

try (AsynchronousFileChannel channel ="input.txt"))) {

ByteBuffer buffer = ByteBuffer.allocateDirect(1024 * 1024 * 100);

Future<Integer> result =, 0);

while(!result.isDone()) {

// Do some other useful work....


System.out.println("Bytes read: " + result.get());


Callback-Based Style

The callback style for asynchronous I/O is based on a CompletionHandler, which defines two methods, completed() and failed(), that will be called back when the operation either succeeds or fails.

This style is useful if you want immediate notification of events in asynchronous I/O—for example, if there are a large number of I/O operations in flight, but failure of any single operation is not necessarily fatal:

byte[] data = {2, 3, 5, 7, 11, 13, 17, 19, 23};

ByteBuffer buffy = ByteBuffer.wrap(data);

CompletionHandler<Integer,Object> h =

new CompletionHandler() {

public void completed(Integer written, Object o) {

System.out.println("Bytes written: " + written);


public void failed(Throwable x, Object o) {

System.out.println("Asynch write failed: "+ x.getMessage());



try (AsynchronousFileChannel channel ="primes.txt"),

StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

channel.write(buffy, 0, null, h);

Thread.sleep(1000); // Needed so we don't exit too quickly


The AsynchronousFileChannel object is associated with a background thread pool, so that the I/O operation proceeds, while the original thread can get on with other tasks.

By default, this uses a managed thread pool that is provided by the runtime. If required, it can be created to use a thread pool that is managed by the application (via an overloaded form of, but this is not often necessary.

Finally, for completeness, let’s touch upon NIO’s support for multiplexed I/O. This enables a single thread to manage multiple channels and to examine those channels to see which are ready for reading or writing. The classes to support this are in the java.nio.channels package andinclude SelectableChannel and Selector.

These nonblocking multiplexed techniques can be extremely useful when writing advanced applications that require high scalability, but a full discussion is outside the scope of this book.

Watch Services and Directory Searching

The last class of asynchronous services we will consider are those that watch a directory, or visit a directory (or a tree). The watch services operate by observing everything that happens within a directory—for example, the creation or modification of files:

try {

WatchService watcher = FileSystems.getDefault().newWatchService();

Path dir = FileSystems.getDefault().getPath("/home/ben");

WatchKey key = dir.register(watcher,




while(!shutdown) {

key = watcher.take();

for (WatchEvent<?> event: key.pollEvents()) {

Object o = event.context();

if (o instanceof Path) {

System.out.println("Path altered: "+ o);






By contrast, the directory streams provide a view into all files currently in a single directory. For example, to list all the Java source files and their size in bytes, we can use code like:

try(DirectoryStream<Path> stream =

Files.newDirectoryStream(Paths.get("/opt/projects"), "*.java")) {

for (Path p : stream) {

System.out.println(p +": "+ Files.size(p));



One drawback of this API is that this will only return elements that match according to glob syntax, which is sometimes insufficiently flexible. We can go further by using the new Files.find() and Files.walk() methods to address each element obtained by a recursive walk through the directory:

final Pattern isJava = Pattern.compile(".*\\.java$");

final Path homeDir = Paths.get("/Users/ben/projects/");

Files.find(homeDir, 255,

(p, attrs) -> isJava.matcher(p.toString()).find())

.forEach(q -> {System.out.println(q.normalize());});

It is possible to go even further, and construct advanced solutions based on the FileVisitor interface in java.nio.file, but that requires the developer to implement all four methods on the interface, rather than just using a single lambda expression as done here.

In the last section of this chapter, we will discuss Java’s networking support and the core JDK classes that enable it.


The Java platform provides access to a large number of standard networking protocols, and these make writing simple networked applications quite easy. The core of Java’s network support lives in the package, with additional extensibility provided by (and in particular,

One of the easiest protocols to use for building applications is HyperText Transmission Protocol (HTTP), the protocol that is used as the basic communication protocol of the Web.


HTTP is the highest-level network protocol that Java supports out of the box. It is a very simple, text-based protocol, implemented on top of the standard TCP/IP stack. It can run on any network port, but is usually found on port 80.

URL is the key class—it supports URLs of the form http://, ftp://, file://, and https:// out of the box. It is very easy to use, and the simplest example of Java HTTP support is to download a particular URL. With Java 8, this is just:

URL url = new URL("");

try (InputStream in = url.openStream()) {

Files.copy(in, Paths.get("output.txt"));

} catch(IOException ex) {



For more low-level control, including metadata about the request and response, we can use URLConnection to give us more control, and achieve something like:

try {

URLConnection conn = url.openConnection();

String type = conn.getContentType();

String encoding = conn.getContentEncoding();

Date lastModified = new Date(conn.getLastModified());

int len = conn.getContentLength();

InputStream in = conn.getInputStream();

} catch (IOException e) {

// Handle exception


HTTP defines “request methods,” which are the operations that a client can make on a remote resource. These methods are called:


Each has slightly different usages, for example:

§ GET should only be used to retrieve a document and NEVER should perform any side effects.

§ HEAD is equivalent to GET except the body is not returned—useful if a program wants to quickly check whether a URL has changed.

§ POST is used when we want to send data to a server for processing.

By default, Java always uses GET, but it does provide a way to use other methods for building more complex applications; however, doing so is a bit involved. In this next example, we’re using the search function provided by the BBC website to search for news articles about Java:

URL url = new URL("");

String rawData = "q=java";

String encodedData = URLEncoder.encode(rawData, "ASCII");

String contentType = "application/x-www-form-urlencoded";

HttpURLConnection conn = (HttpURLConnection) url.openConnection();



conn.setRequestProperty("Content-Type", contentType );




OutputStream os = conn.getOutputStream();

os.write( encodedData.getBytes() );

int response = conn.getResponseCode();

if (response == HttpURLConnection.HTTP_MOVED_PERM

|| response == HttpURLConnection.HTTP_MOVED_TEMP) {

System.out.println("Moved to: "+ conn.getHeaderField("Location"));

} else {

try (InputStream in = conn.getInputStream()) {

Files.copy(in, Paths.get("bbc.txt"),




Notice that we needed to send our query parameters in the body of a request, and to encode them before sending. We also had to disable following of HTTP redirects, and to treat any redirection from the server manually. This is due to a limitation of the HttpURLConnection class, which does not deal well with redirection of POST requests.

In most cases, when implementing these types of more advanced HTTP applications, developers would usually use a specialist HTTP client library, such as the one provided by Apache, rather than coding the whole thing from scratch using JDK classes.

Let’s move on to look at the next layer down the networking stack, the Transmission Control Protocol (TCP).


TCP is the basis of reliable network transport over the Internet. It ensures that web pages and other Internet traffic are delivered in a complete and comprehensible state. From a networking theory standpoint, the protocol properties that allow TCP to function as this “reliability layer” for Internet traffic are:

Connection based

Data belongs to a single logical stream (a connection).

Guaranteed delivery

Data packets will be resent until they arrive.

Error checked

Damage caused by network transit will be detected and fixed automatically.

TCP is a two-way (or bidirectional) communication channel, and uses a special numbering scheme (TCP Sequence numbers) for data chunks to ensure that both sides of a communication stream stay in sync. In order to support many different services on the same network host, TCP uses port numbers to identify services, and ensures that traffic intended for one port does not go to a different one.

In Java, TCP is represented by the classes Socket and ServerSocket. They are used to provide the capability to be the client and server side of the connection respectively—meaning that Java can be used both to connect to network services, and as a language for implementing new services.

As an example, let’s consider reimplementing HTTP. This is a relatively simple, text-based protocol. We’ll need to implement both sides of the connection, so let’s start with a HTTP client on top of a TCP socket. To accomplish this, we will actually need to implement the details of the HTTP protocol, but we do have the advantage that we have complete control over the TCP socket.

We will need to both read and write from the client socket, and we’ll construct the actual request line in accordance with the HTTP standard (which is known as RFC 2616). The resulting code will look something like this:

String hostname = "";

int port = 80;

String filename = "/index.html";

try (Socket sock = new Socket(hostname, port);

BufferedReader from = new BufferedReader(

new InputStreamReader(sock.getInputStream()));

PrintWriter to = new PrintWriter(

new OutputStreamWriter(sock.getOutputStream())); ) {

// The HTTP protocol

to.print("GET " + filename +

" HTTP/1.1\r\nHost: "+ hostname +"\r\n\r\n");


for(String l = null; (l = from.readLine()) != null; )



On the server side, we’ll need to receive possibly multiple incoming connections. To handle this, we’ll need to kick off a main server loop, then use accept() to take a new connection from the operating system. The new connection then will need to be quickly passed to a separate handler class, so that the main server loop can get back to listening for new connections. The code for this is a bit more involved than the client case:

// Handler class

private static class HttpHandler implements Runnable {

private final Socket sock;

HttpHandler(Socket client) { this.sock = client; }

public void run() {

try (BufferedReader in =

new BufferedReader(

new InputStreamReader(sock.getInputStream()));

PrintWriter out =

new PrintWriter(

new OutputStreamWriter(sock.getOutputStream())); ) {

out.print("HTTP/1.0 200\r\nContent-Type: text/plain\r\n\r\n");

String line;

while((line = in.readLine()) != null) {

if (line.length() == 0) break;



} catch(Exception e) {

// Handle exception




// Main server loop

public static void main(String[] args) {

try {

int port = Integer.parseInt(args[0]);

ServerSocket ss = new ServerSocket(port);

for(;;) {

Socket client = ss.accept();

HTTPHandler hndlr = new HTTPHandler(client);

new Thread(hndlr).start();


} catch (Exception e) {

// Handle exception



When designing a protocol for applications to communicate over TCP, there’s a simple and profound network architecture principle, known as Postel’s Law (after Jon Postel, one of the fathers of the Internet) that should always be kept in mind. It is sometimes stated as follows: “Be strict about what you send, and liberal about what you will accept.” This simple principle means that communication can remain broadly possible in a network system, even in the event of quite imperfect implementations.

Postel’s Law, when combined with the general principle that the protocol should be as simple as possible (sometimes called the KISS principle), will make the developer’s job of implementing TCP-based communication much easier than it otherwise would be.

Below TCP is the Internet’s general-purpose haulage protocol—the Internet Protocol (IP) itself.


IP is the “lowest common denominator” transport, and provides a useful abstraction over the physical network technologies that are used to actually move bytes from A to B.

Unlike TCP, delivery of an IP packet is not guaranteed, and a packet can be dropped by any overloaded system along the path. IP packets do have a destination, but usually no routing data—it’s the responsiblity of the (possibly many different) physical transports along the route to actually deliver the data.

It is possible to create “datagram” services in Java that are based around single IP packets (or those with a UDP header, instead of TCP), but this is not often required except for extremely low-latency applications. Java uses the class DatagramSocket to implement this functionality, although few developers should ever need to venture this far down the network stack.

Finally, it’s worth noting some changes that are currently in-flight in the addressing schemes that are used across the Internet. The current version of IP that is in use is IPv4, which has a 32-bit space of possible network addresses. This space is now very badly squeezed, and various mitigation techniques have been deployed.

The next version of IP (IPv6) is coming but it is not widely used yet. However, in the next 10 years, IPv6 should become much more widespread, and the good news is that Java already has good support for the addressing scheme it introduces.