JEP 254: Compact Strings

Summary

Adopt a more space-efficient internal representation for strings.

Goals

Improve the space efficiency of the String class and related classes while maintaining performance in most scenarios and preserving full compatibility for all related Java and native interfaces.

Non-Goals

It is not a goal to use alternate encodings such as UTF-8 in the internal representation of strings. A subsequent JEP may explore that approach.

Motivation

The current implementation of the String class stores characters in a char array, using two bytes (sixteen bits) for each character. Data gathered from many different applications indicates that strings are a major component of heap usage and, moreover, that most String objects contain only Latin-1 characters. Such characters require only one byte of storage, hence half of the space in the internal char arrays of such String objects is going unused.

Description

We propose to change the internal representation of the String class from a UTF-16 char array to a byte array plus an encoding-flag field. The new String class will store characters encoded either as ISO-8859-1/Latin-1 (one byte per character), or as UTF-16 (two bytes per character), based upon the contents of the string. The encoding flag will indicate which encoding is used.

String-related classes such as AbstractStringBuilder, StringBuilder, and StringBuffer will be updated to use the same representation, as will the HotSpot VM's intrinsic string operations.

This is purely an implementation change, with no changes to existing public interfaces. There are no plans to add any new public APIs or other interfaces.

The prototyping work done to date confirms the expected reduction in memory footprint, substantial reductions of GC activity, and minor performance regressions in some corner cases.

For further detail, see:

• State of String Density Performance
• String Density Impact on SPECjbb2005 on SPARC

Alternatives

We tried a "compressed strings" feature in JDK 6 update releases, enabled by an -XX flag. When enabled, String.value was changed to an Object reference and would point either to a byte array, for strings containing only 7-bit US-ASCII characters, or else a char array. This implementation was not open-sourced, so it was difficult to maintain and keep in sync with the mainline JDK source. It has since been removed.

Testing

Thorough compatibility and regression testing will be essential for a change to such a fundamental part of the platform.

We will also need to confirm that we have fulfilled the performance goals of this project. Analysis of memory savings will need to be done. Performance testing should be done using a broad range of workloads, ranging from focused microbenchmarks to large-scale server workloads.

We will encourage the entire Java community to perform early testing with this change in order to identify any remaining issues.

Risks and Assumptions

Optimizing character storage for memory may well come with a trade-off in terms of run-time performance. We expect that this will be offset by reduced GC activity and that we will be able to maintain the throughput of typical server benchmarks. If not, we will investigate optimizations that can strike an acceptable balance between memory saving and run-time performance.

Other recent projects have already reduced the heap space used by strings, in particular JEP 192: String Deduplication in G1. Even with duplicates eliminated, the remaining string data can be made to consume less space if encoded more efficiently. We are assuming that this project will still provide a benefit commensurate with the effort required.