Type Annotations in Java 8

2. Java 7: annotations on declarations @Override public boolean equals(Object obj) @Entity class MyPojo implements Serializable Java 8: annotations on any uses of types @Encrypted String data List<@NonNull String> strings MyGraph = (@Immutable Graph) tmpGraph; Copyright ©2014 FinLingua, Inc. 2

4. Java 7: Overrides @Override protected boolean displaySensitiveInfo() ... } Problem: dynamic dispatch is tricky Solution: have a tool (the compiler) check inheritance automatically Copyright ©2014 FinLingua, Inc. 4

5. Java 7: Persistence @Entity @Table(name="tbl_flight") public class Flight implements Serializable { @Id public Long getId() { return id; } ... } Problem: DB mappings are redundant Solution: have a tool (e.g., Hibernate) create mappings automatically Copyright ©2014 FinLingua, Inc. 5

8. Type Annotations on Any Uses of Types (JSR 308) @Encrypted String data List<@NonNull String> strings MyGraph = (@Immutable Graph) tmpGraph; class UnmodifiableList<T> implements @ReadOnly List<@ReadOnly T> {} Copyright ©2014 FinLingua, Inc. 8

10. Type Annotations Don’t Affect Execution File file = ...; @Encrypted File encryptedFile = ...; // These lines call the same method connection.Send(file); connection.Send(encryptedFile); Copyright ©2014 FinLingua, Inc. 10

16. Run-time Type Checking Use Aspect Oriented Programming (AOP) to insert run-time checks Byte[] read(@Open MyFile this){ // Inserted by the AOP tool if (!this.isOpen()){ throw new IllegalArgumentException(...); } ... } Refined Claim: Type annotations don’t, on their own, affect execution Copyright ©2014 FinLingua, Inc. 16

17. Static Type Checking Prevent run-time exceptions at compile-time: List<String> xs = ...; String x = xs.get(0); // Known to be valid ... = x.Trim(); // Method known to exist int sum = xs.get(1) + 3; // Compiler error ... = x.Foo(); // Compiler error NullPointerException! Copyright ©2014 FinLingua, Inc. 17

18. Type Annotations Qualify Types List<@NonNull String> xs = ...; @NonNull String x = xs.get(0); ... = x.Trim(); // OK Other ways to qualify the String type: @Encrypted String @Format({FLOAT, INT}) String @Localized String Copyright ©2014 FinLingua, Inc. 18

19. Type Checking Annotations Annotations are just syntax, tools give them their semantics (meaning) Java 8 compiler does not check the annotations Java provides a Pluggable Annotation Processing API and Java Compiler API Copyright ©2014 FinLingua, Inc. 19

20. Type Checking via Subtyping @MaybeTainted @Untainted userInput = dbQuery; // Safe dbQuery = "SELECT * FROM " + userInput; // Invalid! @MaybeTainted String userInput; @Untainted String dbQuery; Copyright ©2014 FinLingua, Inc. 20

21. The Checker Framework: Pluggable Type-Checking • Many built-in checkers: null pointers, locking, security, string syntax (regex, format strings), internationalization, ... • Quickly build your own checker • Uses Java 8 type annotations (or comments) – List<@Nullable String> – List</*@Nullable*/ String> • http://checkerframework.org Copyright ©2014 FinLingua, Inc. 21

22. Copyright ©2014 FinLingua, Inc. 22 void nullSafe( MyObject nonNullByDefault, @Nullable MyObject mightBeNull ){ // Smart defaults nonNullByDefault.Foo(); // Safe mightBeNull.Foo(); // Unsafe! if (mightBeNull != null){ // Flow-sensitive mightBeNull.Foo(); // Safe } } Low Annotation Overhead

23. Our Experience • Checkers reveal important latent bugs –Ran on >3 million LOC of real-world code –Found hundreds of user-visible bugs • Mean 2.6 annotations per kLOC • Quickly build your own checkers Copyright ©2014 FinLingua, Inc. 23

25. Type System Brainstorming 1. Runtime Behavior to Prevent Don’t send unencrypted data over the network 2. Legal Operations Only pass encrypted data to send(...) 3. Types of Data Encrypted, Unencrypted @MaybeEncrypted @Encrypted Copyright ©2014 FinLingua, Inc. 25

26. Error-Checking with Type Annotations Support Checker Framework Full support, including annotations in comments Eclipse Null error analysis support IntelliJ IDEA Can write custom inspectors, no null error analysis support No Support PMD Coverity Find Bugs No Java 8 support Check Style No Java 8 support Copyright ©2014 FinLingua, Inc. 26

28. Metaprogramming Aspect Oriented Programming • AspectJ: @Aspect, @Pointcut Dependency Injection • Spring: @Autowired, @Required • Guice: @Inject Persistence • Hibernate/JPA: @Entity Copyright ©2014 FinLingua, Inc. 28

29. Fine-Grained Dependency Injection @Autowired private Store<Product> s1; @Autowired private Store<Service> s2; Spring 4 considers generics a form of qualifier: Type Annotations would allow further refinement: @Autowired private Store<@Prod(Type.Grocery) Product> s1; Copyright ©2014 FinLingua, Inc. 29

30. Fine-Grained Aspect Oriented Programming Annotations on local variables: Copyright ©2014 FinLingua, Inc. 30 // Trace all calls made to the ar object @Trace AuthorizationRequest ar = ... Refine Join-Points: void showSecrets(@Authenticated User user);

31. EnerJ: Approximate Computing Model Specify which data is non-critical: @Approx int pixel Run-time can approximate that data (e.g., convergence criteria) Checker ensures approximate data doesn’t flow into precise expressions Copyright ©2014 FinLingua, Inc. 31

32. Java 8: Annotations on any Uses of Types Annotations are just syntax, tools give them their semantics (meaning) Complementary Goals: 1. Error Checking: quality 2. Metaprogramming: productivity Copyright ©2014 FinLingua, Inc. 32

33. References • The Checker Framework: http://checkerframework.org/ • MCO trajectory: ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf • Werner Dietl et al. Building and using pluggable type- checkers. 2011. • Marc Eaddy and Alfred Aho. Statement Annotations for Fine- Grained Advising. 2006. • Adrian Sampson et al. EnerJ: Approximate Data Types for Safe and General Low-Power Computation. 2011. Copyright ©2014 FinLingua, Inc. 33