Java Keeps Throttling Up!

Rémi ForaxRémi Forax
StarringStarring
José PaumardJosé Paumard

var local type var inference
10

var local type var inference
10
11 var as lambda parameters raw string nestmatescondy
inner classes
private in VM

11 var as lambda parameters nestmatescondy
inner classes
private in VM
12 expr switch
type switch record
const lambda

11 var as lambda parameters nestmatescondy
inner classes
private in VM
12 expr switch
type switch record
const lambda
13+
sealed interface
pattern matching
value type
raw string

Public Poll!
You can get Kahoot! to participate in the poll, just before
the coffee break

Les bons artistes copient,
les grands artistes volent

Anatomy of a Java Distribution
Language Tools
– compiler: javac, ecj
– jshell, jlink, javadoc, javap, jdb, etc
Runtime Environment
– A class library: OpenJDK, Harmony (discontinuated)
– A virtual Machine: Hotspot, J9
●
Bytecode compiler: c1 / c2, graal (java), falcon (llvm)
●
Garbage collector: CMS / G1 / ZGC, shenandoah, c4

Graal
A Java bytecode compiler written in Java
JVM Compiler Interface (JEP 243)
– Interface between the VM and the bytecode compiler
– Hotspot and J9 support JVMCI (since Java 9)
But Graal recompiles itself at the same time it compiles the
application ?

Graal JIT + AOT
Two configurations
– Just in Time Compiler (like c1 and c2)
– Ahead of Time Compiler (like any C compilers)
Graal compiles itself ahead of time (since april 2019)
– libgraal (shared library)
– Clean separation of the memory resource

Trade off between JIT vs AOT
JIT knows runtime informations
– Exact CPU version
– Application Profiling (delayed JIT)
– Can not use slow optimizations
AOT knows all the codes
– Fast startup time, lower runtime overhead
– No dynamic support (no dynamic classloading, no bytecode generation)
– Can not use optimizations that generates too much assembly codes

What About the JIT Performances ?
Let us take a well-known benchmark:
The computation of the Mandelbrot set
Easy!

Julia Sets
Comes from the Julia set:
For a given value of c, this is the set of z0
For which zn+1 = zn
2 + c is bound
If |zn| becomes greater than 2
Then z0 does not belong to
the Julia set

Mandelbrot Set
Set of c for which
z0 = 0
zn+1 = zn
2 + c
Is bound

Computing the Mandelbrot Set
Make each point of an image a complex number c
Compute the following:
z0 = 0
zn+1 = zn
2 + c
If for some n, |zn| becomes greater than 2
Then c does not belong to the Mandelbrot set

Implementation: take an image of size 64x64
For each pixel z
If n reaches 1000 and |zn| stays lower than 2
Then the point belongs to the set
So there are about 16M multiplications to be made
All the multiplications are different

Complex provides a useful abstraction
for (var col = -32; col < +32; col++) {
for (var row = -32; row < +32; row++) {
var c = new Complex(col, row);
var zn = new Complex(0.0, 0.0);
var iteration = 0;
while (zn.modulus() < 4 && iteration < max) {
zn = zn.square().add(c);
iteration++;
}
if (iteration < max) {
image.setRGB(col, row, colors[iteration]);
} else {
image.setRGB(col, row, black);
}
}
}

Computing the Mandelbrot set C style
var an = 0.0; var bn = 0.0;
var iteration = 0;
while (an*an + bn*bn < 4 && iteration < max) {
var next_a = an*an - bn*bn + col;
var next_b = 2*an*bn + row;
an = next_a; bn = next_b;
iteration++;
}
} else {
}
}
}

JIT Performance with JMH
Time to render Mandelbrot 64x64
With jdk 11 (c1 + c2)
score error
primitive 1540 µs/op 22 µs/op
reference 2816 µs/op 48 µs/op

JIT Performance with JMH
Time to render Mandelbrot 64x64
libgraal (ce-rc15) -XX:+UseJVMCICompiler
score error

GraalVM: a Java 8 Distribution
Language Tools
– compiler: javac, ecj
– jshell, jlink, javadoc, javap, jdb, native-image, etc
Runtime Environment
– A class library: OpenJDK + Truffle
– A virtual Machine: Hotspot
●
Bytecode compiler: c1 / c2, graal (java)
●
Garbage collector: CMS / G1 / ZGC
– A C backend optimizer: llvm

native-image
Precompile a JVM-based application into one executable
Use Graal AOT + SubstrateVM
SubstrateVM
– Low memory footprint VM
– No dynamic linking support
– Use low pause non generationnal GC
Support Java, Scala, Kotlin, Clojure

Truffle
Write an interpreter with some specialization machinery to help
the partial evaluator (Graal), get a compiler for free
Truffle is a Java library
– runs on Hotspot or Substrate VM
– ask Graal to aggressively optimize the code
– Allows (dynamic) languages interoperability at runtime

In a Nutshell
A Java 8 application can be executed on:
1) HotSpot with C1 / C2 as JIT compilers
2) HotSpot with Graal as JIT compiler
3) A Jlink generated JRE with HotSpot
4) A native image, AOT compiled with Graal,
executed on SubstrateVM

What Do You Need to Compare ?
How much time it takes to link the application ?
The size of the application ?
How fast does it execute ?

Link-time, run-time & storage
scan rt.jar and compute stats
link-time run-time storage
jdk11 - 1.70 s 288 k (304 M)
jdk8 + libgraal - 1.32 s 288 k (878 M)
jdk11 + jlink 2.2 s 1.53 s 43 M
native-image 22 s (43s) 0.79 s 10 M

OpenJDK Project Metropolis
Rewrite Hotspot in Java
Use libgraal as JIT to replace c1/c2
Rewrite the runtime in Java
Interpreter in Java ?
Rewrite the GCs in Java
Need a JVMGCI ?

Stack allocation in C
C allows stack allocation
typedef struct _complex {
double re;
double im;
} complex;
complex c; // allocate re and im on stack
Abstraction with no allocation cost !

Aliasing and Stack
If structs are passed as pointers, the compiler doesn’t know
if it’s the same address is in memory or not
void foo(complex* c1, complex* c2) {
c1 -> re = 5;
// does c2 -> re has been changed or not ?
}
In Java, you can not create an address on stack
Because the VM re-arranges the stack frames at runtime

Value class
A simple immutable class tagged as value
public value class Complex {
private final double re;
private final double im;
public Complex(double re, double im) {
this.re = re;
this.im = im;
}
public Complex add(Complex c) {
return new Complex(re + c.re, im + c.im);
}
}

Value type
Immediate value with no header
– Immutable and final
– No identity (no address in heap)
– Non null
– Can implement interfaces
Motto: “Code like a class, works like an int”

Back to the Mandelbrot Set
Complex provides a useful abstraction
var c = new Complex(row, col);
var zn = new Complex(0.0, 0.0);
var iteration = 0;
while (zn.modulus() < 4 && iteration < max) {
zn = zn.square().add(c);
iteration++;
}
} else {
}
}
}

JMH
How much time to render the Mandelbrot 64x64 with 1000
iterations?
With java -XX:+EnableValhalla
score error
value 1541 µs/op 12 µs/op

Optional/Option
public value class Option<E> {
private final E value;
private Option(E value) {
this.value = value;
}
public static <E> Option<E> empty() {
return Option<E>.default;
}
public static <E> Option<E> of(E value) {
Objects.requireNonNull(value);
return new Option<>(value);
}
public static <E> Option<E> ofNullable(E value) {
return (value == null)? empty(): of(value);
}
...
All fields are 0, null, 0.0, etc

Benchmark with JMH
private static final Map<String, Integer> MAP = Map.of("foo", 1, "bar", 2, "baz", 3);
private static final List<String> LIST = List.of("a", "foo", "b", "bar", "c", "baz");
@Benchmark
public int sum_null() {
int sum = 0;
for(var key: LIST) {
var value = MAP.get(key);
sum += (value != null)? value: 0;
}
return sum;
}
@Benchmark
public int sum_optional() {
int sum = 0;
var value = Optional.ofNullable(MAP.get(key));
sum += value.orElse(0);
}
return sum;
}

Benchmark with JMH
public final class value Option<E> {
private final E value;
private Option(E e) {
this.e = e;
}
public Option<E> of(E e) {
Objects.requireNonNull(e);
return new Option<>(e);
}
public static <E> Option<E> empty() {
return Option<E>.default;
}
public static <E> Option<E> ofNullable(E value) {
return (value == null)? empty(): of(value);
}
/* rest of the class */
}
@Benchmark
public int sum_option() {
int sum = 0;
var value = Option.ofNullable(MAP.get(key));
sum += value.orElse(0);
}
return sum;
}

JMH
Time to compute the sum
score error
sum_null 60.9 ns/op 0.9 ns/op
sum_optional 71.6 ns/op 3.6 ns/op
sum_option 61.3 ns/op 0.4 ns/op

Classes are not immutable
Problem: a class is not truly immutable
– You can see the field being initialized in the constructor
Only initialize through static factory
– Add 2 new opcodes: default/with
– Let the compiler do the lifting
But not binary compatible => bridges ??

Constructor are static factory
public value class IntBox {
private final int value;
private IntBox(int value) {
this.value = value;
}
public int intValue() {
return value;
}
public static IntBox zero() {
return IntBox.default;
}
public static IntBox valueOf(int value) {
var box = IntBox.default;
box = __WithField(box.value, value);
return box;
}
} WithField allows to change one field

Flattenable
Storing a value type in a field or an array
– Less pointer chasing!
Problems
– with volatile field
●
don’t flatten if volatile
– When writing without synchronized or volatile
●
value type tearing :(

Array flattening ?
Array of value types are flattenable
– Don’t flatten if too big !
header header
header
header
header
An array of java.lang.Integer An array of IntBox

Benchmark
Sum of an array of value type
private static final IntBox[] ARRAY = new IntBox[100_000];
static {
range(0, ARRAY.length).forEach(i -> ARRAY[i] = IntBox.valueOf(i));
}
@Benchmark
public int sum_IntBox() {
var sum = 0;
for(var value : ARRAY) {
sum += value.intValue();
}
return sum;
}

JMH
Time to sum an array of 100 000 values
score error
primitive (int) 27.1 µs/op 0.2 µs/op
reference (Integer) 60.7 µs/op 0.6 µs/op
Value (IntBox) 27.0 µs/op 0.1 µs/op

Benchmark
What if we shuffle the values ?
private static final IntBox[] ARRAY = new IntBox[100_000];
static {
range(0, ARRAY.length).forEach(i -> ARRAY[i] = IntBox.valueOf(i));
Collections.shuffle(Arrays.asList(ARRAY));
}
@Benchmark
public int sum_IntBox() {
var sum = 0;
for(var value : ARRAY) {
sum += value.intValue();
}
return sum;
}

JMH
Time to sum an array of 100 000 random values
score error
primitive (int) 27.1 µs/op 0.2 µs/op
reference (Integer) 121.9 µs/op 5.1 µs/op
Value (IntBox) 27.0 µs/op 0.1 µs/op

First Prototype
Minimum Value Type
– Reference Object can be
unboxed to a value type
– Value type are not subtype of
Object
●
Need to box first

Second Prototype
L-World
– Subtypes of Object
doesn’t work exactly like an int
:(
Minimum Value Type
– Reference Object can be
unboxed to a value type
– Value type are not subtype of
Object
●
Need to box first

L-world issues
Operations that requires an object header
● synchronized throws IllegalStateMonitorException
●
==
– opt-in to component-wise comparison or
– always false
Arrays are covariant
●
Array access are polymorphic :(
– The JIT try to aggressively hoist the element check

L-world issues : Generics
Erased so no flattening
●
Reify generics over value types
A type parameter can be null
●
Need a nullable value type for old generics
Complex?, IntBox?

L-world Issues : Concurrency
Value types are subjects of tearing
public value class SecurityToken {
private final long id1, id2;
public SecurityToken(long id1, long id2) {
if (id1 != id2) throw new IllegalArgumentException();
...
}
public void check() {
if (id1 != id2) throw new IllegalStateException();
}
public static void main(String[] args) {
var tokens = new SecurityToken[1];
IntStream.range(0, 2).forEach(id -> {
new Thread(() -> {
for(;;) {
tokens[0].check();
tokens[0] = new SecurityToken(id, id);
}
}).start();
});
} }

Votation
inline class vs immediate class

Poll!
1. Inline class!
2. Immediate class!
public immediate class Complex { … }
public inline class Complex { … }

Coffee (or whatever)
Break!
Coffee (or whatever)
Break!

Erlang Actor
Scheduled by the Erlang VM (BEAM)
counter(Sum) ->
receive % wait for some message
{increment} ->
counter(Sum+1); % tail-call
{value} ->
io:format("~p~n", [Sum]);
end.
pid ! {increment}; % send a message to pid

Thread are heavyweight !
A Thread
– Share the same address space
– Lightweight Lock
●
no context switch
but
– Thread stack is pre-reserved (=> OOM if a lot of threads)
– Thread scheduling is done by the OS !

Concurrency models
continuation model
Memory
OS
VM
T1 T2 T3
lock
thread model
Memory
OS
VM
T1 T2
lock
C1 C2 C3

Continuation
Wrap a Runnable
Continuation.run() execute the runnable
– Can stop itself with a yield !
– A call to run() will restart the runnable
just after the yield
No language support needed

A simple example
Create a continuation and run it
var scope = new ContinuationScope("example");
var continuation = new Continuation(scope, () -> {
System.out.println("hello Devoxx France");
});
System.out.println("start continuation");
continuation.run();

A simple example
Create a continuation and call run(), yield(), run()
var scope = new ContinuationScope("example");
var continuation = new Continuation(scope, () -> {
System.out.println("hello Devoxx France");
Continuation.yield(scope);
System.out.println("i’m back !");
});
System.out.println("start continuation");
continuation.run();
System.out.println("restart continuation");
continuation.run();

How does it work ?
Yield copy the all stack frames on heap !
main
run
f1
f2
heap
run
f1
f2

How does it work ?
Then run() move some stack frames back
main
f2
heap
stack bang!
The VM tries to guess
how many stack frames
should be moved
A read of the stack bang
trigger the copy of the
remaining frames

Delimited Continuation vs Thread
Scheduling explicit (yield, run)
– No OS context switch
No heap reservation
– Only store what is actually needed !

Java Actor ?
Scheduled by the JVM/JDK
var actor = new Actor(new Runnable() {
int sum;
public void run() {
while (true) {
receive(message -> { // wait for some message
switch((String)message {
case "increment" ->
sum++; // side effect
case "value" -> {
System.out.println(sum);
exit();
}
}});
}
}});
actor.send("increment"); // send a message to the actor

Fiber
Wrap a Runnable
Scheduled using an ExecutorService
– A fiber is scheduled on a thread
Blocking calls (read, write, sleep)
– Freeze the fiber, schedule another one
– Re-scheduled
●
When a read/write/locks/etc will not block
●
Maybe scheduled on another thread

Fiber vs Continuation
A fiber delegates the suspension (yield) to the continuation
– All blocking calls do internally a yield
– The JDK code calls run() on the continuation
when it can be rescheduled
All the JDK needs to be patched to be “fiber aware”

Fiber/Continuation limitation
Continuation/Fiber
– Can not yield if there is a native frame on the stack
Fiber
– Yielding inside a synchronized block pins the fiber to that thread
●
The same thread as to be used when re-scheduling
●
This limitation may be removed in the future

Thread related API ?
Some Thread related APIs need to be reworked
Thread.currentThread()
– Need to lazily create a fake thread per fiber :(
ThreadLocal/InheritableThreadLocal
– Redirect thread local to fiber locals
Will consume a lot of memory

Roadmap
Tailcall support ?
– Not implemented yet
Performance ?
– Still a prototype !
No version of Java targeted, yet !

Precise type inference
Groovy (with @CompileStatic) change the type of a variable after
an instanceof
Object x = ...
if (x instanceof String) {
print(x.length()); // x is a String here !
}
// x is not a String here !

Instanceof in Java
Extends the intanceof syntax to define a new variable
Object x = ...
if (x instanceof String s) {
System.out.println(s.length());
}
Avoid to introduce an intersection type
if (x instanceof I || x instanceof J) { … }

Declaration/Definition
The rules for declaration/definition are a little surprising
class Point {
private final int x;
private final int y;
…
@Override
public boolean equals(Object o) {
if (!(o instanceof Point p) {
return false;
}
return x == p.x && y == p.y;
}
...
}
declaration
definition

Declaration/Definition
Other examples
if (x instanceof String s && s.length() == 3) { …
if (x instanceof String s || foo()) { /* no s here */ }
(x instanceof String s)? s.length(): /* no s here */
while(x instanceof String s) {
… s.length()
}

Enhanced instanceof
Not yet scheduled for a peculiar
version of Java

Data storage class
Sometimes a class is just a storage for data
In Java, you get encapsulation by default,
So you need to declare fields, write boring constructors, boring getters,
boring equals, hashCode and toString

While in Kotlin
you have data class
data class User(val name: String, val age: Int)
the compiler generates
constructor, getters/setters, equals(), hashCode() and
toString()

Known problems
Data class in Kotlin
Can be mutable
So storing data class is a HashSet/HashMap
is a mess
You can still declare fields
There are not used in equals/hashCode/toString ??
The boilerplate code is still there on disk

Record in Java
record User(String name, int age)
A record (a data carrier)
is non mutable
generates overridable
●
Constructor, accessors: String name(), int age()
●
equals/hashCode/toString
– Implementation is provided by invokedynamic
has no supplementary fields

An example
The primary constructor can specify pre-conditions
public record User(String name, int age) {
public User {
Objects.requireNonNull(name);
}
public boolean isOld() {
return age > 14;
}
}

Record vs other features
A record can be combined with a value type
value record User(String name, int age)
the keyword “value” acts as a modifier
Records will have a special support for pattern matching
(using extractors)

Abstract data type in Haskell
A function is defined as a list of pattern matchs
data Expr = Value Int | Add Expr Expr
eval :: Expr -> Int
eval (Value v) = v
eval (Add l r) = eval(l) + eval(r)

Rough translation in Java
New features: sealed interface + switch expression
sealed interface Expr {
record Value(int v) implements Expr;
record Add(Expr l, Expr r) implements Expr;
}
int eval(Expr expr) {
return switch(expr) {
case Value(var v) -> v;
case Add(var l, var r) -> eval(l) + eval(r);
};
}

What’s wrong with switch ?
String[] tokens = ...
Vehicle vehicle;
switch(tokens[0]) {
case "car":
case "sedan":
var color = tokens[1];
vehicle = new Car(color);
break;
case "bus":
vehicle = new Bus("yellow");
break;
default:
throw new …
}

What’s wrong with switch ?
Vehicle vehicle;
switch(tokens[0]) {
case "car":
case "sedan":
break;
case "bus":
break;
default:
throw new …
}
fallthrough
weird scope
initialization hidden

Fix the switch
Separate the cases with a comma + arrow syntax
Vehicle vehicle;
switch(tokens[0]) {
case "car", "sedan" -> {
}
case "bus" -> {
}
default -> throw new …
}

Add a switch expression
A switch that can “return” a value
var tokens = ...
var vehicle = switch(tokens[0]) {
...
}
case "bus" -> new Bus("yellow");
};

Exhaustive Switch
The switch expression requires a “default”
var tokens = ...
...
}
};

Exhaustive Switch (again)
For enums, default is not required if exhaustive
enum Kind { CAR, SEDAN, BUS }
var tokens = ...
var vehicle = switch(kind) {
case CAR, SEDAN -> {
...
}
case BUS -> new Bus("yellow");
};

Return a value from a block
And not return from the method
var tokens = ...
??? new Car(color);
}
};

Return a value from a block
And not return from the method
var tokens = ...
break new Car(color);
}
};

Hyphen keyword
Using ‘-’ to extend the number of keywords !
var tokens = ...
break-with new Car(color);
}
};

Switch enhancement
Retrofit old switch
– Arrow syntax and comma separated case values work for both switch
Introduce switch expression
– exhaustive
– default not needed for enums

Switch enhancement
Already in Java 12
– Under the flag --enable-preview
Not a preview feature in Java 13
– Uses break-with instead of break

String concatenation
A simple benchmark
class PerfTest {
static int value = 4;
@Benchmark
public String test_concat() {
return "err: " + value + " !";
}
@Benchmark
public String test_builder() {
return new StringBuilder().append("err: ")
.append(value).append(" !").toString();
}
@Benchmark
public String test_format() {
return String.format("err: %d !", value);
}
}

JMH
With Java 11
String.format() is super slow !
score error
test_concat 0.013 µs/op 0.001 µs/op
test_builder 0.014 µs/op 0.001 µs/op
test_format 0.502 µs/op 0.034 µs/op

Generated bytecodes
The format parsing is super slow + boxing/varargs
public static java.lang.String format(int);
0: ldc #10 // String err: %d !
2: iconst_1
3: anewarray #11 // class java/lang/Object
6: dup
7: iconst_0
8: iload_0
9: invokestatic #12 // Integer.valueOf:(I)LInteger;
12: aastore
13: invokestatic #13 // String.format:(LString;[LObject;)LString;
16: areturn

JMH
With amber-compiler intrinsics
it’s not slow anymore !
score error
test_concat 0.014 µs/op 0.001 µs/op
test_builder 0.014 µs/op 0.001 µs/op
test_format 0.014 µs/op 0.001 µs/op

Amber / intrinsics
The compiler uses invokedynamic instead
public static java.lang.String format(int);
0: iload_0
1: invokedynamic #10, 0 // #1:format:(I)LString;
6: areturn
bootstraps
1: #35 invokeStatic IntrinsicFactory.staticStringFormatBootstrap
(LLookup;LString;LmethodType;LString;)LCallSite;
Method arguments:
#36 err: %d !

Limitations
Only works if
– the format is a constant!
– the format has no dependency on java.util.Locale!
(double formatting)
In the work
– should work for JIT constant not only compiler constant
– Consider the Locale as a constant + deoptimisation

Example in Java
Java class initialization is lazy by default
=> so not issue here!
But static block initialization is the devil
Usually trigger other class initializations
=> applications are slooow to boot

Use Scala’s lazy keyword
class Utils {
public static final lazy Path HOME =
Path.of(System.getenv("HOME"));
}
Problem:
Scala uses the Double-checked Locking pattern

The Double-checked Pattern
The correct implementation disables further useful optimizations :(
class Utils {
private static volatile Path HOME;
public static Path getHome() {
var home = HOME; // volatile read
if (home == null) {
synchronized(Utils.class) {
home = HOME;
if (home == null) {
return HOME = Path.of(System.getenv("HOME"));
}
}
}
return home;
}
}

Constant Pool
A classfile has a dictionnary of all constants (constant pool), each
constant is transformed once to a runtime Java Object
#30: ...
#31: Utf8: "hello riviera dev"
#32: String: #31
#33: ...
class Utils {
//System.out.println("hello riviera dev");
ldc #32
...
}
}

Constant Dynamic
ConstandDynamic (Java 11) is a constant that can be computed dynamically
#28: Utf8: "Ljava/nio/file/Path;"
#29: Utf8: "HOME"
#30: NameAndType: #29:#28 // HOME Ljava/nio/file/Path;
#31: MethodHandle 6:#… // Utils.lazy_HOME_init ()Ljava/nio/file/Path;
#32: ConstantDynamic: 00:#30
BootstrapMethods:
00: #31
class Utils {
private static Path lazy_HOME_init() { return Path.of(System.getenv("HOME")); }
//System.out.println(??);
ldc #32
...
}
}

Compilation Strategies
A lazy static field
Loaded from the same class
●
Use ldc
Loaded from another class
●
Use getstatic ?

getstatic
Opcode getstatic need to be changed to recognize attribute DynamicValue
#30: NameAndType: #29:#28 // HOME Ljava/nio/file/Path;
#31: MethodHandle 6:#… // Utils.lazy_HOME_init ()Ljava/nio/file/Path;
#32: ConstantDynamic: 00:#30
#33: Field …:#30 // Utils.HOME Ljava/nio/file/Path;
BootstrapMethods:
00: #31
class Utils {
field: #30
DynamicValue: #32
private static Path lazy_HOME_init() { return Path.of(System.getenv("HOME")); }
//System.out.println(??);
getstatic #33
...
}
}

Lazy static : Java vs Scala
Scala (double-checked pattern)
– Guarantee singleton
– Perf issue: Volatile read + nullcheck
Java (getstatic + constant dynamic)
– Singleton after creation
●
need to be idempotent
– Plain static final read
●
constant for the VM

Lazy static final in Java
Scheduled to be introduced in Java 14
“lazy” can not be used on final instance field

Example in Java
Embedded code is unreadable
var html =
"<html>n" +
" <body style="width: 100vw">n" +
" <p>Hello World.</p>n" +
" </body>n" +
" <script>console.log("loadedn")</script>n" +
"</html>";

Example in Java
Embedded code has a lot of boilerplate
var html =
"<html>n" +
" <body style="width: 100vw">n" +
" <p>Hello World.</p>n" +
" </body>n" +
" <script>console.log("loadedn")</script>n" +
"</html>";

Borrow C# raw string
var html =
@"<html>
<body style=""width: 100vw"">
<p>Hello World.</p>
</body>
<script>console.log(""loadedn"")</script>
</html>";
but with a different delimiter
– C# needs to de-specialized quotes
– we can not embed C# easily in Java

Proposed Solution – Fat String
Use """ to start and end a fat strings
var html =
"""<html>
<body style="width: 100vw">
<p>Hello World.</p>
</body>
<script>console.log("loadedn")</script>
</html>""";

Examples
With pattern matching
var pattern = Pattern.compile("""""");
A multi-lines string
var s = """this is a long
multi-lines
of text""";
Embedded code
var s = """< a href="javascript: alert('hello')">""";

Fat/Raw String in Java
Was initially scheduled for Java 11
Was a real raw String (no escape of u….)
Using a variable number of backticks (`)
But early adopter reactions were “meeh”
Still in discussion
Raw ? Auto-alignment ?

Java Keeps Throttling Up!

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