Read all text from a file
Java 11 added the readString() method to read small files as a String
, preserving line terminators:
String content = Files.readString(path, StandardCharsets.US_ASCII);
For versions between Java 7 and 11, here's a compact, robust idiom, wrapped up in a utility method:
static String readFile(String path, Charset encoding)
throws IOException
{
byte[] encoded = Files.readAllBytes(Paths.get(path));
return new String(encoded, encoding);
}
Read lines of text from a file
Java 7 added a convenience method to read a file as lines of text, represented as a List<String>
. This approach is "lossy" because the line separators are stripped from the end of each line.
List<String> lines = Files.readAllLines(Paths.get(path), encoding);
Java 8 added the Files.lines()
method to produce a Stream<String>
. Again, this method is lossy because line separators are stripped. If an IOException
is encountered while reading the file, it is wrapped in an UncheckedIOException
, since Stream
doesn't accept lambdas that throw checked exceptions.
try (Stream<String> lines = Files.lines(path, encoding)) {
lines.forEach(System.out::println);
}
This Stream
does need a close()
call; this is poorly documented on the API, and I suspect many people don't even notice Stream
has a close()
method. Be sure to use an ARM-block as shown.
If you are working with a source other than a file, you can use the lines()
method in BufferedReader
instead.
Memory utilization
The first method, that preserves line breaks, can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), and the decoded characters (each of which is 16 bits even if encoded as 8 bits in the file) reside in memory at once. It is safest to apply to files that you know to be small relative to the available memory.
The second method, reading lines, is usually more memory efficient, because the input byte buffer for decoding doesn't need to contain the entire file. However, it's still not suitable for files that are very large relative to available memory.
For reading large files, you need a different design for your program, one that reads a chunk of text from a stream, processes it, and then moves on to the next, reusing the same fixed-sized memory block. Here, "large" depends on the computer specs. Nowadays, this threshold might be many gigabytes of RAM. The third method, using a Stream<String>
is one way to do this, if your input "records" happen to be individual lines. (Using the readLine()
method of BufferedReader
is the procedural equivalent to this approach.)
Character encoding
One thing that is missing from the sample in the original post is the character encoding. There are some special cases where the platform default is what you want, but they are rare, and you should be able justify your choice.
The StandardCharsets
class defines some constants for the encodings required of all Java runtimes:
String content = readFile("test.txt", StandardCharsets.UTF_8);
The platform default is available from the Charset
class itself:
String content = readFile("test.txt", Charset.defaultCharset());
Note: This answer largely replaces my Java 6 version. The utility of Java 7 safely simplifies the code, and the old answer, which used a mapped byte buffer, prevented the file that was read from being deleted until the mapped buffer was garbage collected. You can view the old version via the "edited" link on this answer.
Tips about writing micro benchmarks from the creators of Java HotSpot:
Rule 0: Read a reputable paper on JVMs and micro-benchmarking. A good one is Brian Goetz, 2005. Do not expect too much from micro-benchmarks; they measure only a limited range of JVM performance characteristics.
Rule 1: Always include a warmup phase which runs your test kernel all the way through, enough to trigger all initializations and compilations before timing phase(s). (Fewer iterations is OK on the warmup phase. The rule of thumb is several tens of thousands of inner loop iterations.)
Rule 2: Always run with -XX:+PrintCompilation
, -verbose:gc
, etc., so you can verify that the compiler and other parts of the JVM are not doing unexpected work during your timing phase.
Rule 2.1: Print messages at the beginning and end of timing and warmup phases, so you can verify that there is no output from Rule 2 during the timing phase.
Rule 3: Be aware of the difference between -client
and -server
, and OSR and regular compilations. The -XX:+PrintCompilation
flag reports OSR compilations with an at-sign to denote the non-initial entry point, for example: Trouble$1::run @ 2 (41 bytes)
. Prefer server to client, and regular to OSR, if you are after best performance.
Rule 4: Be aware of initialization effects. Do not print for the first time during your timing phase, since printing loads and initializes classes. Do not load new classes outside of the warmup phase (or final reporting phase), unless you are testing class loading specifically (and in that case load only the test classes). Rule 2 is your first line of defense against such effects.
Rule 5: Be aware of deoptimization and recompilation effects. Do not take any code path for the first time in the timing phase, because the compiler may junk and recompile the code, based on an earlier optimistic assumption that the path was not going to be used at all. Rule 2 is your first line of defense against such effects.
Rule 6: Use appropriate tools to read the compiler's mind, and expect to be surprised by the code it produces. Inspect the code yourself before forming theories about what makes something faster or slower.
Rule 7: Reduce noise in your measurements. Run your benchmark on a quiet machine, and run it several times, discarding outliers. Use -Xbatch
to serialize the compiler with the application, and consider setting -XX:CICompilerCount=1
to prevent the compiler from running in parallel with itself. Try your best to reduce GC overhead, set Xmx
(large enough) equals Xms
and use UseEpsilonGC
if it is available.
Rule 8: Use a library for your benchmark as it is probably more efficient and was already debugged for this sole purpose. Such as JMH, Caliper or Bill and Paul's Excellent UCSD Benchmarks for Java.
Best Answer
This is modified code so it's support dynamic column size. The idea is to create a row once and reuse it if the row is already exists.
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