October 23, 2020

After Java 8: Part 2 — Java 9


This is the second in a series of articles about changes in Java ecosystem after version 8. In the previous article, we covered changes in licensing and release cadence. In this article, we will discuss changes in the language itself that came with version 9.

Java 9

Java 9 is released in September 2017 and it is non LTS release, with the end of support scheduled for March 2018, it brought probably the biggest changes to Java ecosystem in the last 10 years.

Module system

A huge amount of credits for making Java 9 release with the biggest changes in the last 10 years go to the module system. It was initially planned for Java 7, but postponed for 8 and subsequently to 9. It is one of the biggest changes in Java ever, since it does not affect only language itself but also a compiler, VM, and Tooling.

Module system is a result of project Jigsaw. The first and main reason for this feature is to modularize JDK itself, make it maintainable again. Second (and to be honest a lot smaller) reason is to enable developers to modularize their own applications. Of course, modularizing applications is purely optional, but still, they run on modular JDK.
JDK becoming too large is the main motivation behind the module system. Now, there are more than 90 platform modules.


Easy unless:
1. You or some libs use JDK types that have been encapsulated.
They will still run, but they will not compile… This can change in the next versions. If you want, you can enforce encapsulation in runtime by passing the following flag to jvm:--illegal-access=deny You can bypass encapsulation at compile time by using -add-exports $moduleYouNeed/$packageYouNeed=$yourModule
This command line option exports $packageYouNeed of $moduleYouNeed to $yourModule. Code in $yourModule can hence access all public types in $packageYouNeed but other modules can not.
This option is available for the java and javac commands. Although it is for now not necessary for java command, since as stated app will still run. But again, this is something that can change in the future.

When setting $yourModule to ALL-UNNAMED, all code from the classpath can access that package. This is a preferred way to go when accessing internal APIs during migration to Java 9. Example:
javac --add-exports java.base/

If you have trouble migrating there is a handy tool shipped with JDK to help you. jdeps -jdkinternals target/classes/com/mbrkljac/javaaftereight/HttpClientDemo.class Using this tool you can check dependencies without running or compiling. It also suggests solutions for potential problems it finds.

2. You or some lib use types from non-default Java modules ( This can be mitigated using: javac --add-modules java.xml.bind and java --add-modules java.xml.bind Main

More info about jdeps:

Module system implementation

Module has a name, it groups related code as well as resources and is fully self-contained.
Every module has a module descriptor which consists of:

  • Module’s name
  • Module’s dependencies (that is, other modules this module depends on)
  • Packages it explicitly makes available to other modules (all other packages in the module are implicitly unavailable to other modules)
  • services it offers
  • services it consumes
  • to what other modules it allows reflection

A module descriptor is the compiled version of a module declaration that’s defined in a file named Module descriptor is stored in a file named module-info.class in the module’s root folder. Each module declaration begins with the keyword module, followed by a unique module name and a module body enclosed in braces, as in:

module modulename {}

The module declaration’s body can be empty or may contain various module directives like:

  • requires. A requires module directive specifies that this module depends on another module — this relationship is called a module dependency. Each module must explicitly state its dependencies. When module A requires module B, module A is said to read module B and module B is read by module A. To specify a dependency on another module, use requires, as in:

requires modulename;

  • requires transitive — implied readability. To specify a dependency on another module and to ensure that other modules reading your module also read that dependency — known as implied readability — use requires transitive, as in:

requires transitive modulename;

  • exports and exports…to. An exports module directive specifies one of the module’s packages whose public types (and their nested public and protected types) should be accessible to code in all other modules. An exports…to directive enables you to specify in a comma-separated list precisely which module’s or modules’ code can access the exported package — this is known as a qualified export.
  • uses. A uses module directive specifies a service used by this module — making the module a service consumer. A service is an object of a class that implements the interface or extends the abstract class specified in the uses directive.
  • provides…withA provides…with module directive specifies that a module provides a service implementation — making the module a service provider. The provides part of the directive specifies an interface or abstract class listed in a module’s uses directive and the with part of the directive specifies the name of the service provider class that implements the interface or extends the abstract class.
  • open, opens, and opens…toBefore Java 9, reflection could be used to learn about all types in a package and all members of a type — even its private members — whether you wanted to allow this capability or not. Thus, nothing was truly encapsulated.
    A key motivation of the module system is strong encapsulation. By default, a type in a module is not accessible to other modules unless it’s a public type and you export its package. You expose only the packages you want to expose. With Java 9, this also applies to reflection, allowing runtime-only access to a package. An opens module directive of the form opens package
    indicates that a specific package’s public types (and their nested public and protected types) are accessible to code in other modules at runtime only. Also, all the types in the specified package (and all of the types’ members) are accessible via reflection.
  • Allowing runtime-only access to a package by specific modules. An opens…to module directive of the form opens package to comma-separated-list-of-modules indicates that a specific package’s public types (and their nested public and protected types) are accessible to code in the listed modules at runtime only. All of the types in the specified package (and all of the types’ members) are accessible via reflection to code in the specified modules.
  • Allowing runtime-only access to all packages in a module. If all the packages in a given module should be accessible at runtime and via reflection to all other modules, you may open the entire module, as in: open module modulename {// module directives}

The keywords exports, module, open, opens, provides, requires, uses, with, as well as to and transitive, are restricted keywords. They’re keywords only in module declarations and may be used as identifiers anywhere else in your code.

Cyclic dependencies are not supported.


  • Security (Module can expose some of its packages, and encapsulate other)
  • Reduced footprint
  • Easy deprecation (In java 9, java.corba was removed)
  • Future proof (Incubator modules jdk.incubator.httpclient)
  • Improved performance — The JVM uses various optimization techniques to improve application performance. JSR 376 indicates that these techniques are more effective when it’s known in advance that required types are located only in specific modules.


JShell is a command line tool that provides interactive use of Java Programming Language elements. JShell is a REPL(Read -> type code; Eval -> execute code; Print -> see results; Loop -> interactively refine).
With it, we can enter program elements one at a time, immediately seeing the result and adjusting accordingly. It is mainly intended to be used to test out ideas, explore API’s as well as teaching.

Main features:

  • It’s just plain java.
  • You get code completion,
  • it has built-in documentation.
  • It is possible to incrementally create code inside of jshell. (variable can be declared later)
  • It is possible to save and open snippets
  • Snippets can be edited using an external editor
  • JShell API enables IDE integration. (IntelliJ: Tools/JShell Console)

Library and Language Improvements

Java 9 comes with some small extensions to existing java libraries. There are no major changes like in java 8, changes in module system are big enough for themselves.

Collection factory methods

Old ways:

List food  = new ArrayList<>();

List food = Arrays.asList("Lamb")
List food = new ArrayList<>()
List food = Collections.emptyList()

New way:
List food = List.of("Lamb", "Pork", "Beef")
Set strings = Set.of("One", "Two")
Map map = Map.of("One", 1, "Two", 2)

Those methods return immutable collections and are heavily optimized. Note that iteration order for Set and Map is not guaranteed. (It was not guaranteed before also, but with the old approach, the order was mostly consistent. Now, that is not the case.)

Stream API Improvements

Added methods:
static Stream ofNullable(T t)

List nullableList = getStringsPossibleNull();
//Old wayStream strings = nullableList == null ? Stream.empty() :;//New way
Stream strings = Stream.offNullable(nullableList);

Stream takeWhile(Predicate predicate)
TakeWhile is similar to filter in the sense that it expects a predicate and returns a new stream consisting only of the elements that match the given predicate. In an ordered stream, takeWhile takes elements from the initial stream while the predicate holds true. Meaning that when an element is encountered that does not match the predicate, the rest of the stream is discarded.
If the stream is unordered, if some of the elements in the stream match the predicate (but not all) then the operation is nondeterministic and an arbitrary subset of matching elements is returned or removed. If all elements match the predicate, all input will be returned. If none matches, the empty stream will be returned.

jshell> Stream.of(2, 4, 5, 6 ,8)
.takeWhile(n -> n % 2 == 0)
// prints out: 24jshell> Set numbers = Set.of(2, 4, 5, 6 ,8);
.takeWhile(n -> n % 2 == 0)
//Prints out different result depending on how Set has been initialisednumbers ==> [2, 8, 6, 5, 4]

Stream dropWhile(Predicate predicate) dropWhile is the opposite of takeWhile. It drops the element until the first element that matches the predicate, and includes the rest in the result stream;

static Stream iterate(T seed, Predicate hasNext, UnaryOperator next) Overloaded iterate method, added hasNext parameter which is used to determine when the Stream must terminate.

jshell> Stream.iterate(0, i -> i < 9, i -> i + 1)
//Prints out: 012345678

New Collectors:


The Collectors.filtering is similar to the Stream filter(); it’s used for filtering input elements but used for different scenarios. The Stream’s filter is used in the stream chain, whereas the filtering is a Collector that was designed to be used along with groupingBy.

With Stream’s filter, the values are filtered first and then it’s grouped. In this way, the values which are filtered out are gone and there is no trace of it. If we need a trace then we would need to group first and then apply filtering which actually the Collectors.filtering does.

The Collectors.filtering takes a function for filtering the input elements and a collector to collect the filtered elements:

List numbers = List.of(1, 2, 3, 5, 5);

Map result =
.filter(val -> val > 3)
.collect(Collectors.groupingBy(i -> i, Collectors.counting()));

result =
.collect(Collectors.groupingBy(i -> i,
Collectors.filtering(val -> val > 3, Collectors.counting())));


The Collectors.flatMapping is similar to Collectors.mapping but has a more fine-grained objective. Both the collectors take a function and a collector where the elements are collected but flatMapping function accepts a Stream of elements which is then accumulated by the collector.

Additions to optional

  • ifPresentOrElse
    public void ifPresentOrElse(Consumer action, Runnable emptyAction)
    Optional value = Optional.of("someValue"); value2.ifPresentOrElse(v -> System.out.println(v), () -> System.out.println("empty"));
  • or
    Used when we want to execute some other action that also returns an Optional.
    Prior to Java 9, the Optional class had only the orElse() and orElseGet() methods but both need to return unwrapped values.
    Java 9 introduces the or() method that returns another Optional lazily if our Optional is empty. If our first Optional has a defined value, the lambda passed to the or() method will not be invoked, and value will not be calculated and returned:
    Optional result = value.or(() -> defaultValue);
  • stream
    Added to achieve interoperability between Optional and Stream. Calling stream() on empty optional will result in an empty stream.
    Quite useful in combination with Stream.flatMap
List> listOfOptionals = Arrays.asList(Optional.empty(), Optional.of("foo"),
 Optional.empty(), Optional.of("bar"));  
 // Java 8:List filteredList =
   .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
   .collect(Collectors.toList());//Java 9:  
`List filteredList =

java.time improvements

  • long dividedBy(Duration divisor)
  • Duration truncatedTo(TemporalUnit unit)
  • static Clock systemUTC() -> can be accurate in nanoseconds depending on platform.
  • Stream datesUntil(LocalDate endExclusive, Period step) && Stream datesUntil(LocalDate endExclusive)

Other small improvements

  • Single underscore as an identifier is now illegal.
  • Try-with-resources improvements With the old approach, a new variable was required:
public void doWithFileInputStream(FileInputStream fis) {
try(FileInputStream fis2 =fis) {;

New approach:

public void doWithFileInputStream(FileInputStream fis) {
try(fis) {;

Note: in order to use a variable in try statement, it needs to be effectively final. Effectively final variable is variable that either has the final keyword or is not reassigned within the method.

New APIs. Process API and Http2 API

Process API

java.lang.Process Represents native processes created from Java java.lang.ProcessHandle Represents any native process on the operating system

ProcessHandle.allProcesses() -> Returns a stream of a current snapshot of processes state…

ProcessHandleInfoDemo ProcessHandleDestroyDemo

HttpClient API

Supports HTTP/2 and WebSocket
Caveat: incubator module (They can change, and are not by default part of jdk.)
In order to use it pass following flag:
 --add-modules jdk.incubator.httpclient

Reactive Streams (Flow API)

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  • Not meant as end-user API
  • Stream data with support for backpressure
  • Vendor-neutral specification (
  • Flow API: interfaces added to JDK
  • Interoperability for reactive projects like RxJava, Akka Streams
  • HttpClient implements Publisher/Subscriber interfaces
  • java.util.concurrent.Flow support is implemented in RxJava 2, Spring 5, Akka Streams

StackWalker API

Old way:
StackTraceElement[] stackTrace = Thread.getStackTrace()

  • Low performance
  • No guarantee all stack elements are returned
  • No partial handling possible

New way:

StackWalker walker = StackWalker.getInstance();


Performance and security improvements

G1 Garbage Collector

GC combinations deprecated in Java 8 were removed. CMS Collector is deprecated

  • G1 (garbage first) garbage collector is introduced in java 6 and is now default when JVM is running in server mode (as opposed as running in client mode)

To read more about server vs client JVM:

Important commands to remember for tuning G1:


Concurrent Mark Sweep Heap structure:

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G1 Heap structure:

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G1 Heap allocation:

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Read more:

String performance

  • Compact strings
  • Lower memory usage without any code changes.
  • Strings in Java are internally represented by a char[] containing the characters of the String. And, every char is made up of 2 bytes because Java internally uses UTF-16. If a String contains a word in the English language, the leading 8 bits will all be 0 for every char, as an ASCII character can be represented using a single byte.
  • As of Java 9, strings are stored as byte[] and not as char[]
  • Final field coder is added. This field preserves information about encoding.
  • If encoding is LATIN1, single byte will be used to represent every char. In the case of UTF16, 2 bytes will be used.
  • This leads up to 15% less memory consumption.
* The identifier of the encoding used to encode the bytes in
* {@code value}. The supported values in this implementation are
* UTF16
* @implNote This field is trusted by the VM, and is a subject to
* constant folding if String instance is constant. Overwriting this
* field after construction will cause problems.
private final byte coder;

Concatenation of Strings

Before java9:

String nemaRajaBez = "rodnoga " + "kraja";

Would result in:

6:  new           #4      // class StringBuilder
9: dup
10: invokespecial #5 // Method StringBuilder.""
13: aload_1 // String rodnoga
14: invokevirtual #6 // Method StringBuilder.append:(LString;)LStringBuilder;
17: aload_2 // String kraja
18: invokevirtual #6 // Method StringBuilder.append:(LString;)LStringBuilder;21: invokevirtual #7 // Method StringBuilder.toString:()LString;

Java 9:

6: aload_1		  // String rodnoga           
7: aload_2 // String kraja
8: invokedynamic #4, 0 // InvokeDynamic #0:makeConcatWithConstants:
// (LString;LString;)LString;BootstrapMethods:
0: #19 invokeStatic StringConcatFactory.makeConcatWithConstants:
(… , … , LMethodType; , LString; , … ) LCallSite;
Method arguments:
#20 \u0001\u0001

The reason to change the compiler now in this way is, from the project description, to “enable future optimizations of String concatenation without requiring further changes to the bytecode emitted by javac.”. Dynamic method invocation is an ideal solution for that challenge, as it delays method implementation to the runtime. The developers of the Java runtime can then improve the implementation of the factory class, without all other developers needing to recompile their projects.


There are numerous vulnerabilities related to serialization. Java 9 tries to address this by adding a new interface to filter data before deserializing.

interface ObjectInputFilter {
Status checkInput(FilterInput filterInput);
enum Status {

ObjectInoutStream::setObjectInputFilter -> per stream
ObjectInputFilter.Config.setSerialFilter -> for all streams

Alternatively, you can filter incoming serialization data without adding or changing code. For this, you can use jdk.serialFilter system property. This property is backported to Java 6/7/8

maxbytes=n; maxarray=n; maxdepth=n;
com.hybrid.dto.; (All types from this package can be serialized)
; (No types from this package can be serialized)

Further reading: