Day of the Dukentacle : la meilleure façon de prévoir le futur, c'est de le fabriquer soi-même

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Day of the Dukentacle Fabriquons le futur de peur qu’il ne nous fabrique! José Paumard Java Developer Advocate Java Platform Group Rémi Forax Maître de conferences Université Gustave Eiffel

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https://twitter.com/JosePaumard https://github.com/JosePaumard https://www.youtube.com/c/JosePaumard01 https://www.youtube.com/user/java https://www.youtube.com/hashtag/jepcafe https://fr.slideshare.net/jpaumard https://www.pluralsight.com/authors/josepaumard https://dev.java

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https://twitter.com/He bah non! https://github.com/forax https://speakerdeck.com/forax OpenJDK, ASM, Tatoo, Pro, etc… One of the Father of invokedynamic (Java 7) Lambda (Java 8), Module (Java 9) Constant dynamic (Java 11) Record, text blocks, sealed types (Java 14 / 15) Valhalla (Java 23+)

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 4 Covering several new things in the Java language First part: - Unnamed classes (JEP 445, JEP 458) - Unnamed patterns (JEP 456) - String Template (JEP 465) What’s in it for you?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 5 Covering several new things in the Java language Second part: - Statement before super (JEP 447) - Foreign Function & Memory API (JEP 454) What’s in it for you?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 15 Conceptually, an extension of HTML Add 3 kind of attributes - hx-trigger - hx-post, hx-get, hx-delete, etc + hx-vals - hx-target + hx-swap Server-side rendering != react SSR > htmx

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 17 We want to generate HTML/XML hx-trigger - Could be JSON, SQL, etc… - With holes for values from the current context The values should be properly escaped - No SQL injection, etc… The result is not necessary a String String Template Processor

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 19 XML template processor + composition html-component record Product(String name, int price) implements Component { public Renderer render() { return $.""" \{name}\{price * 1.20} """; } } https://github.com/forax/html-component

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 21 Not a String Not a StringTemplate (fragments + values) An interface that - Validates fragments - Interpolates values using the correct escaping String Template Processor

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 23 A plain old static method is more powerful No need for a special syntaxe Long Live StringTemplate Methods! public static Renderer XML(StringTemplate template) { ... }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 25 Separating the validation and the interpolation? - The validation is done only once Where Can We Go? class XMLDOM { static XMLDOM of(StringTemplate template) { ... } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 26 Separating the validation and the interpolation? - The validation is done only once - The interpolation is done using the validated object Where Can We Go? static Renderer XML(XMLDom dom, Object... args) { ... }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 28 Does it have to be the first instruction of a constructor? Call to super() class A { A() { super(); // calls Object() } } class B extends A { B() { super(); // calls A() } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 29 If new B(null), all the fields of A are initialized before B is discovered as invalid It Delays Precondition Checks class Named { String name; Named(String name) { this.name = name; } } class Student extends Named { Student(String name) { super(name); Objects.requireNonNull(name); } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 30 Computation has to be done inside the call to super(...) It Makes the Code Less Readable class Hashed { int hash; Hashed(int hash) { this.hash = hash; } } class Named extends Hashed { String name; Named(String name) { super(Utils.hash(name)); this.name = name; } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 31 Computation has to be done inside the call to super(...) It Makes the Code Less Readable class Hashed { int hash; Hashed(int hash) { this.hash = hash; } } class Named extends Hashed { String name; Named(String name) { var hash = Utils.hash(name) super(hash); this.name = name; } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 32 Javac generates the code before the call to super(…) Code Before super()? interface Expr { int eval(); static Expr add(int a, int b) { return new Expr() { public int eval() { return a + b; } }; } } 0: aload_0 1: iload_1 2: putfield // val$a:I 5: aload_0 6: iload_2 7: putfield // val$b:I 10: aload_0 11: invokespecial // Object."":()V 14: return

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 37 Value types are unmodifiable Valhalla? value class Optional { private final T value; private Optional(T value) { this.value = value; // initialization before super() super(); } static Optional empty() { return new Optional<>(null); } static Optional of(T value) { return new Optional<>(value); } }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 40 It is about accessing the off heap memory This memory is not handled by the Garbage Collector The GC: - Gives you the memory to store your objects - Tracks if what it gives you is still in use or not - Reclaims it if not Drawback: GCs like to move your objects around What is it about?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 41 Why would you store information off heap? - You can store raw data (≠ unstructured) - You can directly store data coming from your I/O (buffers don’t like to be moved around) - You can read and process it very efficiently Why do you need off heap memory?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 42 Two of them: 1) The ByteBuffer API - API from Java NIO (Java 4, 2002) - No type safety - Limited to 2 GB (indexed by an int) - The memory is reclaimed when the ByteBuffer object is garbaged What are the alternatives?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 43 Two of them: 2) The infamous sun.misc.Unsafe API - Unsafe… - Unsafe doesn’t mean fast, it means unsafe The Memory API aims to fix all this What are the alternatives?

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 44 1) It is fast 2) There is no 2 GB limit 3) You can choose between unsafe or type safety 4) You can reclaim the memory on demand 5) You can control the thread safety 6) Access is bound in space and time The Memory API fixes all this

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 47 Then, add some data in this memory segment How is it working? Arena arena = Arena.global(); MemorySegment segment = arena.allocate(80L, 1L); for (long index; index < 10L; index++) { segment.setAtIndex( ValueLayout.JAVA_LONG, index, random.nextLong(0L, 100L)); }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 48 And then, read it back How is it working? for (long index; index < 10L; index++) { var l = segment.getAtIndex( ValueLayout.JAVA_LONG, index); }

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Arena 4/18/2024 Copyright © 2023, Oracle and/or its affiliates 49 MemorySegment MemorySegment MemorySegment int int int int int long int int double MemorySegment int int int MemorySegment int long int int double

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 51 An arena is a factory to create memory segment There are four of them: What is this Arena object? var global = Arena.global(); // singleton var auto = Arena.ofAuto(); var confined = Arena.ofConfined(); var shared = Arena.ofShared();

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 52 An arena is a factory to create memory segment There are four of them: What is this Arena object? Closeable Bounded Lifetime Shared among threads Global No Auto No Confined Yes Shared Yes

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 53 An arena is a factory to create memory segment There are four of them: What is this Arena object? Closeable Bounded Lifetime Shared among threads Global No No Auto No Yes Confined Yes Yes Shared Yes Yes

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 54 An arena is a factory to create memory segment There are four of them: What is this Arena object? Closeable Bounded Lifetime Shared among threads Global No No Yes Auto No Yes Yes Confined Yes Yes Shared Yes Yes

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 55 An arena is a factory to create memory segment There are four of them: What is this Arena object? Closeable Bounded Lifetime Shared among threads Global No No Yes Auto No Yes Yes Confined Yes Yes No Shared Yes Yes Yes

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 56 A MemorySegment is an interface A MemorySegment is a contiguous segment of memory A segment can be: - A heap segment, on-heap - Or a native segment, off-heap More about the MemorySegment public sealed interface MemorySegment { }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 59 You can model the content of a MemorySegment with the MemoryLayout interface Modeling with MemoryLayout public sealed interface MemoryLayout { }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 60 ValueLayout.JAVA_LONG tells that this segement is holding long primitive types Modeling with MemoryLayout for (long index; index < 10L; index++) { segment.setAtIndex( ValueLayout.JAVA_LONG, index, random.nextLong(0L, 100L)); }

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 61 The MemoryLayout interface ValueLayout models all the primitive types sealed interface ValueLayout extends MemoryLayout permits ValueLayout.ofByte, ValueLayout.ofShort, ValueLayout.OfInt, ValueLayout.ofLong ValueLayout.ofFloat, ValueLayout.ofDouble, ValueLayout.ofChar, ValueLayout.ofBoolean { }

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0 4/18/2024 Copyright © 2023, Oracle and/or its affiliates 62 setAtIndex() accesses the indexth int Modeling with MemoryLayout for (long index; index < 10L; index++) { segment.setAtIndex( ValueLayout.JAVA_INT, index, random.nextInt(0L, 100L)); } 1 2

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 63 set() accesses the indexth cell Modeling with MemoryLayout for (long index; index < 10L; index++) { segment.set( // = set at offset ValueLayout.JAVA_INT, index, random.nextInt(0L, 100L)); } 0 1 2

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 64 There two others memory layouts: - a layout to model structs - a layout to model fixed-size arrays - a padding layout - a union layout Other MemoryLayouts

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 66 First define a struct layout Modeling with MemoryLayout var pointLayout = MemoryLayout.structLayout( ValueLayout.JAVA_INT.withName("x"), ValueLayout.JAVA_INT.withName("y"));

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 67 First define a struct layout Why do you need the names x and y? Modeling with MemoryLayout var pointLayout = MemoryLayout.structLayout( ValueLayout.JAVA_INT.withName("x"), ValueLayout.JAVA_INT.withName("y"));

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 68 You can access all these elements using a var handle VarHandle is a class added to Java 9 Added after the MethodHandle class in Java 7 Getting a Varhandle From a MemoryLayout

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 69 1) Define a var handle on any of the field of the struct Defining a Var Handle var pointLayout = MemoryLayout.structLayout( ValueLayout.JAVA_INT.withName("x"), ValueLayout.JAVA_INT.withName("y")); var xHandle = pointLayout.varHandle( MemoryLayout.PathElement.groupElement("x")) .withInvokeExactBehavior();

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4/18/2024 Copyright © 2023, Oracle and/or its affiliates 70 2) Use the var handle to read or write this field Defining a Var Handle var xHandle = pointLayout.varHandle( MemoryLayout.PathElement.groupElement("x")) .withInvokeExactBehavior(); int x = (int) xHandle.get(segment, 0L); xHandle.set(segment, 0L, 1234);

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The Future of Java is Bright!