Java File I/O

The code of the HappyCoders.eu article series on files in Java – one package per article. Every class with a main() method is one chapter of its article and prints the outputs shown there.

Articles

Article	Package
Java BufferedReader (English, German)	eu.happycoders.bufferedreader
Java PrintWriter (English, German)	eu.happycoders.printwriter
Java FileReader (not yet published; German)	eu.happycoders.filereader
Java FileWriter (not yet published; German)	eu.happycoders.filewriter
Serialization in Java (English, German)	eu.happycoders.serialization
Creating, copying, moving, and deleting files and directories (not yet published; German, English)	eu.happycoders.files
Listing directory contents (not yet published; German, English)	eu.happycoders.listing

Running the examples

The examples use Java 25 language features and need Java 25 or newer. Build once with mvn compile, then run a class from the repository root:

java -cp target/classes eu.happycoders.<package>.<ClassName>

Files the examples create go into target/; the serialization examples write their .ser files into the current directory.

Java BufferedReader – eu.happycoders.bufferedreader

Most examples read the small log file files/server.log.

• Intro – reads the log file line by line
• Ch1Creating – Files.newBufferedReader(), FileReader, InputStreamReader around a GZIPInputStream, an own buffer size, and a Latin-1 file read as UTF-8
• Ch2ReadLine – line terminators, empty lines, and the end of the input
• Ch3Lines – lines() with filter() and groupingBy(), an UncheckedIOException while streaming, and Files.lines()
• Ch4ReadAll – readAllLines() and readAllAsString() (Java 25), also after a readLine()
• Ch5Characters – read() and read(char[], int, int)
• Ch6MarkReset – an optional header line with mark() and reset(), and skip()
• Ch6Ready – ready() returns false, yet the next readLine() returns a line
• Ch7Console – reading lines from System.in with the charset from stdin.encoding (Java 25)
• Ch7Readln – IO.readln() (Java 25)
• Ch8Scanner – tokens with Scanner and with split(), and how both classes report an IOException
• Ch9Closing – closing the BufferedReader closes the reader inside it
• Ch10Mistakes – calling readLine() twice per loop iteration

Java PrintWriter – eu.happycoders.printwriter

Most examples write the small log file target/server.log, the file the BufferedReader examples read.

• Intro – writes the log file with printf()
• Ch1Creating – new PrintWriter(String) with and without a charset, a PrintWriter around Files.newBufferedWriter(), and one around a GZIPOutputStream
• Ch1WriteCalls – counts the write calls that arrive below a PrintWriter, with and without a BufferedWriter in between
• Ch1Append – appending to a file: APPEND alone, CREATE and APPEND, and a FileWriter with append = true
• Ch2Print – print() and println() for different types and null, the line separator, write(int) vs. print(int), and append()
• Ch3Printf – format strings, the locale, chained printf() calls, and a missing argument
• Ch4Flush – when the bytes reach the file, with and without autoFlush
• Ch5CheckError – a failing writer, writing after close(), and the euro sign in ISO-8859-1
• Ch6Closing – closing the PrintWriter closes the writer inside it, and what closing a PrintWriter around System.out does
• Ch7StringWriter – text and a stack trace into a string, and the same with formatted() and StringBuilder
• Ch8Console – a PrintWriter around System.out and System.console().writer()
• Ch9PrintStream – how often a PrintStream and a PrintWriter, both with autoFlush, flush their stream
• Ch10Mistakes – a PrintWriter that is never closed leaves the file empty

Java FileReader – eu.happycoders.filereader

Most examples read the small log file files/server.log.

• Intro – reads the log file with readAllAsString() (Java 25)
• Ch1Creating – a FileReader from a file name, a File, and a Path, getEncoding() for different charsets, and files that cannot be opened
• Ch2Read – read() character by character, and an emoji arriving as two halves of a surrogate pair
• Ch2ReadArray – read(char[]) in blocks, and what is left in the array after the last block
• Ch2ReadAll – readAllLines() and readAllAsString() (Java 25), also after skip()
• Ch2TransferTo – re-encoding a file from ISO-8859-1 to UTF-8 with transferTo()
• Ch2Unsupported – mark() and reset() are not supported
• Ch3Replacement – a Latin-1 file read as UTF-8: replacement characters from a FileReader, a MalformedInputException from an InputStreamReader with a reporting decoder
• Ch3Bom – a UTF-8 byte order mark arrives as the first character
• Ch4InputStreamReader – a classpath resource read with an InputStreamReader, and an unknown charset name
• Ch5BufferedReader – a BufferedReader around a FileReader
• Ch6Binary – a binary file read with a FileReader and with a FileInputStream
• Ch8Closing – reading after close(), and open file descriptors of FileReaders that are never closed (Linux and macOS)

Java FileWriter – eu.happycoders.filewriter

Most examples write lines of the log file files/server.log into target/server.log.

• Intro – writes two log lines
• Ch1Creating – a FileWriter from a file name, a File, and a Path, overwriting and appending, appending to a file that does not exist, getEncoding(), and files that cannot be created
• Ch2Write – write(int) with the two halves of a surrogate pair, with a number, and with a code point; write(String) and append(), also with null
• Ch2Flush – when the bytes reach the file: flush(), many short writes through the 512-byte buffer, and one long write that grows it to 8,192 bytes
• Ch3Unmappable – the euro sign in ISO-8859-1: a question mark from a FileWriter, an UnmappableCharacterException from an OutputStreamWriter with a reporting encoder, from Files.newBufferedWriter(), and from Files.writeString(); half a surrogate pair at the end of a string
• Ch4OutputStreamWriter – an OutputStreamWriter around a GZIPOutputStream, and an unknown charset name
• Ch5BufferedWriter – a BufferedWriter around a FileWriter
• Ch6Binary – all 256 byte values written with a FileWriter and with a FileOutputStream
• Ch8Closing – a FileWriter that is never closed leaves the file empty, also after a garbage collection; writing after close()

Serialization – eu.happycoders.serialization

The examples work on a small order model: the classes Order and Customer and the record OrderItem. Chapters that need a variant of a class – one that is not serializable, has no serialVersionUID, or validates in readObject() – keep it in a package of its own (graph, transients, versioning, inheritance, custom, special, filter).

• Intro – writes an order into a file and reads it back
• Ch1Writing – ObjectOutputStream: writing into a byte array, the first bytes of the stream, buffering
• Ch2Reading – ObjectInputStream: pattern matching on the result, several objects in one stream, buffering, a file that is not a serialization stream
• Ch3ObjectGraph – NotSerializableException, shared references, the same object written twice, reset() and writeUnshared()
• Ch4Transient – transient fields after deserialization and how readObject() restores them
• Ch5Versioning – the computed serialVersionUID and what happens when a class changes between writing and reading
• Ch5Misdeclaration – a misdeclared serialVersionUID, for the JFR event jdk.SerializationMisdeclaration (the command is in the class comment)
• Ch6Inheritance – a serializable class with a non-serializable superclass
• Ch7Custom – validation in readObject(), writeObject() for the superclass state, readResolve(), the serialization proxy
• Ch8Special – records, enums (including a renamed constant), lambdas, and inner classes
• Ch8LambdaNames – a serializable lambda written with write and read with read; renaming its variable and recompiling makes the old stream unreadable (the steps are in the class comment)
• Ch9Collections – ArrayList capacity, List.of(), views such as keySet() and subList()
• Ch10Filters – deserialization filters from patterns, the missing !*, and the limits maxdepth (against Joshua Bloch’s deserialization bomb) and maxarray
• Ch10FilterFactory – a filter factory that applies a filter within a ScopedValue scope
• Ch10GlobalFilter – a JVM-wide filter with -Djdk.serialFilter (the command is in the class comment; add -Djava.util.logging.config.file=serialfilter-logging.properties to log rejected classes)
• Ch11Size – the size of an order with ObjectOutputStream and with DataOutputStream

The class PackageSwitchingObjectInputStream in eu.happycoders.serialization.util reads a stream written by one version of a class with another version of it: the chapters keep both versions side by side, e.g. versioning.explicit.v1.Customer and versioning.explicit.v2.Customer. Serialization compares class names without their package, so this simulates a class that changed between writing and reading.

Creating, copying, moving, and deleting files – eu.happycoders.files

Every example starts with a fresh directory target/logs/ that holds a copy of files/server.log. Examples.reset() creates that directory, and Examples.printTree() prints what is in it after an operation. The outputs of the article's run are in results/files/outputs-jdk25.txt. Ch2Exists, Ch3CreateFile, and Ch5TempFiles set or read POSIX permissions and run on Linux and macOS only.

• Intro – rotates the log file: create the directory, move the old log aside, create a new one
• Ch1FilesVsFile – File.renameTo() and File.delete() return false, Files.move() and Files.delete() say why
• Ch2Exists – exists(), notExists(), isRegularFile(), isDirectory(), a dangling symbolic link, and a directory in which existence cannot be determined
• Ch2ExistsRace – two threads check whether a file exists and create it if not, 10,000 times: how often the check was already outdated
• Ch3CreateFile – createFile(), its exceptions, create-if-not-exists, writeString() with CREATE_NEW, POSIX permissions, File.createNewFile()
• Ch4CreateDirectory – createDirectory() vs. createDirectories(), their exceptions, File.mkdir() and mkdirs()
• Ch5TempFiles – createTempFile(), createTempDirectory(), their permissions, cleaning up with try/finally
• Ch6Copy – copy() into a directory, onto an existing file, with COPY_ATTRIBUTES and NOFOLLOW_LINKS, from an InputStream, to an OutputStream, and mismatch()
• Ch7CopyDirectory – copy() on a directory creates only an empty one; copyDirectory() with walkFileTree()
• Ch8Move – move() into a directory, renaming with resolveSibling(), moving a directory, REPLACE_EXISTING, and writing to a temp file followed by ATOMIC_MOVE
• Ch9Delete – delete(), deleteIfExists(), DirectoryNotEmptyException, deleting a symbolic link, File.delete()
• Ch10DeleteDirectory – deleteDirectory() with walkFileTree() and with walk(), and a missing directory
• Ch11Links – createSymbolicLink(), readSymbolicLink(), createLink(), and what survives deleting the original

Listing directory contents – eu.happycoders.listing

The examples list the directory files/project/, a small Maven project with five files in three subdirectories. Directories that some examples create themselves end up under target/.

• Intro – Files.list() on the project directory
• Ch1List – list() is not recursive, the order is the file system's, sorted(), filtering for directories, NotDirectoryException and NoSuchFileException
• Ch1DirectoryStream – newDirectoryStream() with the globs *.md and *.{md,xml}
• Ch1OpenStreams – opens streams without closing them until the process runs out of file handles
• Ch2ListRecursive – finding .java files with list() and recursion written by hand
• Ch3Walk – walk(), its depth-first order, maxDepth 1 and 0, FOLLOW_LINKS, and a FileSystemLoopException
• Ch4WalkFileTree – walkFileTree() with a SimpleFileVisitor, a LoggingVisitor that numbers every call, SKIP_SUBTREE, and TERMINATE
• Ch5Find – find() by file name, by size, and with maxDepth 1
• Ch6PathMatcher – glob:**/*.java, a glob on the file name only, and a regex: matcher
• Ch7AccessDenied – how walk(), find(), walkFileTree() with and without visitFileFailed(), and list() with recursion react to a directory without read permission; sets POSIX permissions and runs on Linux and macOS only
• Ch8LegacyFile – File.list() and File.listFiles(), which return null for a missing directory

The outputs of the article's run are in results/listing/outputs-jdk25.txt.

Benchmarks

Each article's JMH benchmarks are in the benchmark subpackage of its package:

• ReadBenchmark – reading a 100,000,000-byte UTF-8 text file with InputStreamReader, FileReader, and BufferedReader (read(), read(char[]), readLine(), lines(), readAllLines()), and with Scanner.nextLine()
• BufferSizeBenchmark – readLine() with buffer sizes from 256 to 1,048,576 characters
• ReadCallCounter – not JMH: counts how often the InputStreamReader reads from the file when a BufferedReader reads it line by line, for buffer sizes from 256 to 1,048,576 characters; the count is the same for every size
• DecoderWarmupCheck – not JMH: decodes the test file with an InputStreamReader in blocks of one size after warming the JVM up with another, one JVM per call; it shows that the speed depends on the warm-up block size, not on the measured one (the command is in the class comment)
• WriteBenchmark – writing 1,222,747 lines, 100,000,000 bytes of UTF-8 text, with BufferedWriter, with a PrintWriter around Files.newBufferedWriter(), around a file name, and around a FileWriter, with autoFlush, and with a PrintStream; the lines are those of the test file that ReadBenchmark reads
• FormatBenchmark – writing 1,000,000 log lines with printf(), with println() and string concatenation, and with one print() call per field, with and without a BufferedWriter below the PrintWriter
• WriteCallCounter – not JMH: counts the write calls that arrive at the OutputStreamWriter and at the OutputStream below it when a PrintWriter writes the test lines, with and without a BufferedWriter and with autoFlush
• filewriter.benchmark.WriteBenchmark – writing the same 1,222,747 lines with a FileWriter character by character, line by line, and in blocks of 8,192 characters, and with a BufferedWriter around the FileWriter character by character, line by line, and line by line with a flush() after every line
• CopyBenchmark – copies a file of 100,000,000 bytes with Files.copy(), with Files.copy() and COPY_ATTRIBUTES (a clone on APFS since Java 20), with InputStream.transferTo(), with FileChannel.transferTo(), and with a loop that copies through an 8,192-byte buffer from a FileInputStream to a FileOutputStream; the teardown forces each copy to disk before deleting it
• CloneSizeCheck – not JMH: clones files of 1,000,000, 100,000,000, and 1,000,000,000 bytes with Files.copy() and COPY_ATTRIBUTES and prints the median time per size; on APFS it does not depend on the size

mvn package builds target/benchmarks.jar with all of them. Run the benchmarks of one article with its package as filter – a bare class name like ReadBenchmark can match more than you expect:

mvn package
java -jar target/benchmarks.jar eu.happycoders.bufferedreader.benchmark
java -jar target/benchmarks.jar eu.happycoders.printwriter.benchmark
java -jar target/benchmarks.jar eu.happycoders.filewriter.benchmark
java -jar target/benchmarks.jar eu.happycoders.files.benchmark

The BufferedReader and PrintWriter runs take about 15 minutes each, the FileWriter run about ten minutes, the copy benchmark about five minutes. Close other applications while they run. The results of the articles' runs are in results/<package>/, e.g. results/printwriter/apple-m5-pro/.

Additional Resources

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