LTS LoGeek Night in Odessa

1. Back to the future:
Evolution of the Java Type System
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2. Agenda
 History: Java 5 Type System
 Generalized Target-Type Inference
 Annotated Types
 Checker Framework
 Project Valhalla
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3. Java 5 Type System
 Parametric polymorphism
public class Sample<T> {
 Bounded wildcards
 Intersection types
 Type erasure
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private T data;
public T getData() { ... }
}
Collection<? extends Serializable>
public class Box<T extends Comparable & Clonable> { ...

4. Java 8 Type System
 JEP 101: Generalized Target-Type Inference
- Add support for method type-parameter inference in method context
- Add support for method type-parameter inference in chained calls
 JSR 308: Annotations on Java Types
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5. Target Typing
Map<Integer, List<String>> map = new HashMap<>();
...
map.put(key, Collections.emptyList());
public <E> void process(List<E> list, Class<E> type) {...}
...
process(new ArrayList<>(), String.class)
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6. Target Typing
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List<Number> numbers = Arrays.asList(1,2,3);
Iterator<String> iterator = new ArrayList<>().iterator();

7. Annotation Type Target
 ANNOTATION_TYPE
 CONSTRUCTOR
 FIELD
 LOCAL_VARIABLE
 METHOD
 PACKAGE
 PARAMETER
 TYPE
 TYPE_PARAMETER
 TYPE_USE
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Java 8

8. Type annotations usage
 generic type arguments
List<@Localized Document> files;
 type parameter bounds
Collection<? @NotNull Number> files;
 class inheritance
class UnmodifiableList<T> implements @Readonly List<T> {...}
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9. Type annotations location
 object creation
new @Interned MyObject();
 casting
myString = (@NonNull String) myObject;
 throwing exceptions
void update() throws @Critical DataAccessException { ... }
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10. www.luxoft.com
The Java 8 release does not
provide a type checking framework

11. Checker Framework
 Nullness Checker
 Interning Checker
 Lock Checker
 Fake Enum Checker
 Tainting Checker
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 Regex Checker
 Format String Checker
 Property File Checker
 Signature Checker
 Immutability Checker

12. Example 1. NulnessChecker
import org.checkerframework.checker.nullness.qual.Nullable;
public class NullnessExample {
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public String buildMessage(@Nullable String str) {
return "Message: " + str.toLowerCase();
}
}
javac -processor org.checkerframework.checker.nullness.NullnessChecker ...
Compile-time error

13. Example 1. NulnessChecker
import org.checkerframework.checker.nullness.qual.Nullable;
public class NullnessExample1 {
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public String buildMessage(@Nullable String str) {
if (str != null) {
return "Message: " + str.toLowerCase();
}
return "";
}
}
Compiles successfully

14. Example 2. Tainting Checker
import org.checkerframework.checker.tainting.qual.Tainted;
import org.checkerframework.checker.tainting.qual.Untainted;
public class TainedExample {
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public void execute(@Tainted String id) {
String query = buildQuery(id);
//...
}
public String buildQuery(@Untainted String id) {
return "SELECT p.name FROM product p where p.id = " + id;
}
}
javac -processor org.checkerframework.checker.tainting.TaintingChecker ...
Compile-time error

15. Example 2. Tainting Checker
import org.checkerframework.checker.tainting.qual.Tainted;
import org.checkerframework.checker.tainting.qual.Untainted;
public class TainedExample {
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public void execute(@Tainted String id) {
String query = buildQuery(verify(id));
//...
}
public @Untainted String verify(@Tainted String id) {
if (id.matches(".*D.*")) {
throw new IllegalArgumentException("String contains invalid chars!");
}
return new @Untainted String(id);
}
public String buildQuery(@Untainted String id) {
return "SELECT p.name FROM product p where p.id = " + id;
}
}
Compiles successfully

16. Example 3. Immutability Checker
import org.checkerframework.checker.javari.qual.ReadOnly;
import java.util.*;
public class MutabilityExample {
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public void example() {
ImmutableClass obj = new ImmutableClass();
obj.getValues().add("test");
}
private class ImmutableClass {
private List<String> values = new ArrayList<>();
public @ReadOnly List<String> getValues() {
return values;
}
}
}
javac -processor org.checkerframework.checker.javari.JavariChecker ...

17. Project Valhalla
 Value Types
 Generic Specialization
 Reified generics ?
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18. “Codes like a class, works like an int!” ©
Java type system aggregate data types
• heterogeneous aggregates with identity (classes)
• homogeneous aggregates with identity (arrays)
Object identity => footprint and performance costs
Value types: identityless, immutable, pointer-free aggregates
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19. Value Types use cases
 Numerics
 Native types
 Algebraic data types
 Tuples
 Cursors
 Flatting
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20. Generic Specialization
Goal: support generics over primitives and value types
Specialization approaches
• ahead-of-time (Scala)
• on-demand (C#)
class Box<T> {
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private final T t;
public Box(T t) { this.t = t; }
public T get() { return t; }
}

21. Bytecode (current JVM version)
class Box extends java.lang.Object{
private final java.lang.Object t;
public Box(java.lang.Object);
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Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: aload_1
6: putfield #2; //Field t:Ljava/lang/Object;
9: return
public java.lang.Object get();
Code:
0: aload_0
1: getfield #2; //Field t:Ljava/lang/Object;
4: areturn
}

22. Bytecode marked up to preserve erasure information (proposed)
class Box extends java.lang.Object{
private final java.lang.Object*T t;
public Box(java.lang.Object*T);
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Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: aload_1*T
6: putfield #2; //Field t:Ljava/lang/Object*T;
9: return
public java.lang.Object* get();
Code:
0: aload_0
1: getfield #2; //Field t:Ljava/lang/Object*T;
4: areturn*T
}

23. Bytecode after specialization
class Box${T=int} extends java.lang.Object{
private final int t;
public Box${T=int}(int);
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: iload_1
6: putfield #2; //Field t:int;
9: return
public int get();
Code:
0: aload_0
1: getfield #2; //Field t:int;
4: ireturn
}
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24. Links
1. https://www.oracle.com/javaone/sessions/index.html Daniel Smith. Type Inference in Java SE 8.
2. http://2013.javapoint.ru/talks/21/ Александр Ильин. Type annotations in Java 8. И почему это хорошо.
3. http://types.cs.washington.edu/checker-framework/ The Checker Framework
4. http://mail.openjdk.java.net/pipermail/valhalla-dev/2014-July/000000.html Welcome to Valhalla
5. http://cr.openjdk.java.net/~jrose/values/values-0.html John Rose, Brian Goetz, and Guy Steele. State of
the Values. April 2014: Infant Edition
6. http://cr.openjdk.java.net/~briangoetz/valhalla/specialization.html Brian Goetz. State of Specialization.
Jule 2014: Infant Edition
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Q/A