Practical Java Performance: Date Formatting Optimization

Table of Contents


One does not simply measure JVM code performance. A performance engineer must consider numerous variables which affect the resulting data. Several of these are enumerated below:

  • Interpreter and compiled modes (implementation details).
  • Preemptive compiler optimizations based on collected profiling information.
  • Other optimizations such as: constant folding, loop unrolling, dead code elimination.

JMH: The Ultimate Java Benchmarking Tool

Java Microbenchmarking Harness (JMH) is a Java harness for building, running, and analyzing nano/micro/milli/macro benchmarks written in Java and other languages targeting the JVM.

To use this tool, create a maven project from the JMH archetype.

mvn archetype:generate \
          -DinteractiveMode=false \
          -DarchetypeGroupId=org.openjdk.jmh \
          -DarchetypeArtifactId=jmh-java-benchmark-archetype \
          -DgroupId=com.github.raipc \
          -DartifactId=benchmark \

A sample JMH benchmark is shown below:

public class CharIsDigitBenchmark {
    @Param(value = {"+7(955)123-45-67", "79551234567"} )
    private String input;

    public String writeDigitsCharacter() {
        final int length = input.length();
        final StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            char ch = input.charAt(i);
            if (Character.isDigit(ch)) {
        return sb.toString();

    public String writeDigitsString() {
        final String digits = "0123456789";
        final int length = input.length();
        final StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length; i++) {
            String ch = input.substring(i, i+1);
            if (digits.contains(ch)) {
        return sb.toString();

Each instrumented method is annotated with @Benchmark annotation.

The @State annotation marks the class that contains the benchmark state. This annotation defines either the same class as the instrumented methods, or a separate class, in which case the state object is provided to benchmark method as a method parameter.

The @Param annotation marks parameterized fields. The initialization values can be provided via annotation parameters or by using command line parameter -p {param-name}={param-value}.

Understand the principles of writing an informative JMH benchmark by studying the provided Examples.

Date Formatters Performance Comparison

Review date formatter benchmarks on GitHub.

To record benchmarks, build and run the project.

mvn clean package
java -jar target/benchmarks.jar
Benchmark                       Mode      Cnt        Score    Error   Units
parseCustomCurrentATSD        sample  3722617      504.680 ±  8.328   ns/op
parseJoda                     sample  2381165      934.201 ± 15.333   ns/op
parseOptimized                sample  2845261      142.150 ±  4.712   ns/op
printJodaIso8601              sample  3662844      509.362 ±  8.079   ns/op
printWithCustomPrinterIsoOpt  sample  2369326      271.344 ± 10.401   ns/op

The Formatters Used by ATSD

ATSD uses different date parsing and formatting libraries to cover all use cases. The following table clarifies which date formatting libraries are used by each ATSD subsystem.

ATSD Subsystem Date Formatter
Data API joda.time to parse, custom formatter to print.
SQL Apache Commons
Rule Engine joda.time
Forecasts SimpleDateFormat
CSV Parser SimpleDateFormat
User Interface SimpleDateFormat

Each formatter supports different patterns, hence all of them need to be documented, with the differences among them emphasized.

ATSD DatetimeProcessor

The introduction of a new date formatter is to improve overall maintainability by reducing the number of supported libraries, date patterns, documentation notes. After analyzing common use cases, the following API was created.

public interface DatetimeProcessor {
    long parseMillis(String datetime);

    long parseMillis(String datetime, @NotNull ZoneId zoneId);

    ZonedDateTime parse(String datetime);

    ZonedDateTime parse(String datetime, @NotNull ZoneId zoneId);

    String print(long timestamp);

    String print(long timestamp, @NotNull ZoneId zoneId);

DatetimeProcessor API use cases:

DatetimeProcessor fmt = DateTimeFormatterManager.createFormatter(pattern); // for multiple usage of custom format
String datetime = TimeUtils.formatDateTime(millis, pattern, zoneId); // for single usage of custom format
String datetime = TimeUtils.formatDateTime(millis, pattern);
long timestamp = TimeUtils.parseMillis(timestamp, pattern);

long timestamp = TimeUtils.parseWithDefaultParser(datetime); // for tests or best-effort parsing

long timestamp = parseISO8601(String date);
String datetime = formatISO8601(long time);
String datetime = formatISO8601millis(long time);
String datetime = formatLocalNoTimezone(long time);
String datetime = formatLocalMillisNoTz(long time);

Supported Patterns

DatetimeProcessor supports Java 8 DateTimeFormatter patterns with several differences:

  • No need to escape T literal.
  • u pattern is translated to ccccc, day of week starting from Monday.
  • Z pattern is translated to XX, RFC822 offset, Z for zulu.
  • ZZ pattern is translated to XXX, ISO8601 offset, Z for zulu.
  • ZZZ pattern is translated to VV, Zone ID.


DatetimeProcessor interface is implemented by three classes. DatetimeProcessorIso8601 and DatetimeProcessorLocal represent highly optimized date processors for ISO pattern yyyy-MM-ddTHH:mm:ss[.SSSSSSSSS][Z] and local time pattern yyyy-MM-dd HH:mm:ss[.SSSSSSSSS][Z]. DatetimeProcessorCustom is a wrapper for java.time.format.DateTimeFormatter objects. Formatting strictly delegates to default implementation. The parsing operation is performed in semi-manually: After the resolving step, which includes validating, combining and simplifying the various fields into more useful ones, is performed the date object is constructed manually by providing default field values if needed.

By design, DatetimeProcessor objects must not be constructed manually. Instead, use the DateTimeFormatterManager.createFormatter(pattern) factory method.

This method is responsible for a number of tasks:

  1. Attempt to acquire the DatetimeProcessor for the specified pattern from cache.
  2. In case of cache-miss, normalize the pattern.
  3. Insert the most appropriate DatetimeProcessor implementation to cache.


Caching date formatters is not an innovative idea: Previous libraries used this approach under the hood.

  • joda.time cached formatters using ConcurrentHashMap limited by 5000 items.
  • Apache Commons used unlimited ConcurrentHashMap cache.
  • DatetimeProcessor objects are cached in the managed LRU cache dateTimeFormatters limited by cache.formatters.maximum.size property (defaults to 100) which is cleared on demand with the Settings > Cache Management form.

DatetimeProcessor caching method advantages:

  1. Defense from cache pollution.
  2. Cache replacement policy (LRU) demonstrates higher throughput in worst scenarios, which is when many formatters are used.
  3. Size is controlled by the user.

Breaking Good

Here is an example of cache pollution attack:

SELECT *, date_format(time, CONCAT('''time: ''', 'yyyy-MM-dd HH:mm:ss', ''', value: ', value, '''')) AS "time_and_value"
FROM "mpstat.cpu_busy"
LIMIT 500000

The above query only affects date formatting with a dynamic pattern, when DatetimeProcessor is returned by DateTimeFormatterManager. This does not affect the performance of date formatting in Data API or other subsystems.

A better query is shown here:

SELECT *, CONCAT('time: ', date_format(time, 'yyyy-MM-dd HH:mm:ss'), ', value: ', value) AS "time_and_value"
FROM "mpstat.cpu_busy"
LIMIT 500000


Some performance considerations:

  • Use JSR-310 ZoneOffset instead of TimeZone to parse zone offsets which offers free RFC822 offsets support.
  • Manipulate datetime units using OffsetDateTime instead of Calendar.
  • Optimize parseInt function with limited characters support.
  • Implement sizeInDigits function using divide-and-conquer approach.
  • Use JVM intrinsics if possible.


Part 1

Part 2


JMH home page

Aleksey Shipilev's Talk about Java Benchmarking

Tagir Valeev's Talk about JIT Optimizations

SimpleDateFormat Pattern Reference

Joda Time Documentation

DateTimeFormatter Pattern Reference

ATSD Date Format Pattern Reference