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Java 15 Features

Java 15, released in September 2020, brought a wave of exciting features that enhanced developer experience, performance, and security. Java 15 reached general availability in September 2020 and is the next short-term release for the JDK platform. It builds on several features from earlier releases and also provides some new enhancements.

Added isEmpty() Default Method to CharSequence

java.lang.CharSequence has been updated in this release to define a default isEmpty() method that tests if a character sequence is empty. Testing for, and filtering out, empty Strings and other CharSequences is a common occurrence in code and CharSequence::isEmpty can be used as a method reference. Classes that implement java.lang.CharSequence and another interface that defines isEmpty() method should be aware of this addition as they may need to be modified to override the isEmpty() method.

Support for Unicode 13.0

This release upgrades Unicode support to 13.0, which includes the following:

  • The lang.Character class supports Unicode Character Database of 13.0 level, which 13.0 adds 5,930 characters, for a total of 143,859 characters. These additions include 4 new scripts, for a total of 154 scripts, as well as 55 new emoji characters.
  • The java.text.Bidi and java.text.Normalizer classes support 13.0 level of Unicode Standard Annexes, #9 and #15, respectively.
  • The java.util.regex package supports Extended Grapheme Clusters based on 13.0 level of Unicode Standard Annex #29 For more detail about Unicode 13.0.

Hidden Classes

Introduces hidden classes in Java 15. Hidden classes have the following implications to existing code:

Class::getName traditionally returns a binary name, but for a hidden class it returns a string that contains an ASCII forward slash (/) and is therefore not a binary name. Programs that assume the returned string is a binary name might need to be updated to handle hidden classes. That said, the longstanding practice of Unsafe::defineAnonymousClass was to define classes whose names were not binary names, so some programs may already handle such names successfully.

Class::descriptorString and MethodType::descriptorString returns a string that contains a ASCII dot (.) for a hidden class and therefore is not a type descriptor conforming to JVMS 4.3. Programs that assume the returned string is a type descriptor that conforms to JVMS 4.3 might need to be updated to handle hidden classes.

Class::getNestMembers is changed to not throw an exception when it fails to validate a nest membership of any member listed in NestMembers attribute. Instead, Class::getNestMembers returns the nest host and the members listed in the host's NestMembers attribute that are successfully resolved and determined to have the same nest host as this class. (It means it may return fewer members that listed in NestMembers attribute.) Existing code that expects LinkageError when there is a bad nest membership might be impacted.

The nestmate test in the JVM is changed to throw only IllegalAccessError when the nest membership is invalid. Some historical understanding is necessary:

In Java 8, every access control failure was signalled with IllegalAccessError (IAE). Moreover, if a given access check failed with IAE once, then the same check would fail with IAE every time.

In Java 11, the introduction of nest mates (JEP 181) meant that an access control failure could be signalled either with IllegalAccessError or, if nest membership was invalid, LinkageError. Still, if a given access check failed with a specific exception, then the same check would always fail with the same exception.

In Java 15, the introduction of Lookup::defineHiddenClass implies that the nest host of the lookup class must be determined eagerly, when the hidden class is defined as a nestmate of the lookup class. Both Lookup::defineHiddenClass and Class::getNestHost both determine the nest host of a class in a more resilient manner than the JVM did in Java 11; namely, the API simply treats a class as self-hosted if its purported nest membership is invalid. For consistency with the API, the JVM no longer throws LinkageError when a class's nest membership is invalid, and instead treats the class as self-hosted. It means that the JVM only throws IAE from access control (because a self-hosted class will not permit any other class to access its private members). It is the behavior expected by the vast majority of user code.

JVM TI GetClassSignature returns a string that contains a ASCII dot (.) for a hidden class. JVM TI agents might need updating for hidden classes if they assume the returned string from GetClassSignature is a type descriptor conforming to JVMS 4.3.

Added Support for SO_INCOMING_NAPI_ID Support

A new JDK-specific socket option SO_INCOMING_NAPI_ID has been added to in this release. The socket option is Linux specific and allows applications to query the NAPI (New API) ID of the underlying device queue associated with its socket connection and take advantage of the Application Device Queue (ADQ) feature of high performance Network Interface Card (NIC) devices.

Specialized Implementations of TreeMap Class Methods

The TreeMap class now provides overriding implementations of the putIfAbsent(), computeIfAbsent(), computeIfPresent(), compute(), and merge methods. The new implementations provide a performance improvement. However, if the function provided to the compute- or merge methods modifies the map, ConcurrentModificationException may be thrown, because the function that is provided to these methods is prohibited from modifying the map. If a ConcurrentModificationException occurs, the function must either be changed to avoid modifying the map, or the surrounding code should be rewritten to replace uses of the compute- and merge methods with conventional Map methods such as get and put.

Text Blocks

Text blocks have been added to the Java language. A text block is a multi-line string literal that avoids the need for most escape sequences, automatically formats the string in a predictable way, and gives the developer control over the format when desired.

Garbage Collector

In Java 15, both ZGC (JEP 377) and Shenandoah (JEP 379) will be no longer be experimental. Both will be supported configurations that teams can opt to use, while the G1 collector will remain the default. Both were previously available using experimental feature flags. The approach allows developers to test the new garbage collectors and submit feedback without downloading a separate JDK or add-on. One note on Shenandoah: it is not available from all vendor JDKs - most notably, Oracle JDK doesn't include it.


Java 15 builds on several features of past releases, including records, text blocks, new garbage collection algorithms, and more. It also adds new preview features, including sealed classes and hidden classes. As Java 15 is not a long-term-support release, we can expect support for it to end in March 2021. At that time, we can look forward to Java 16, followed soon after with a new long-term-support version in Java 17.

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