Hacking Java @JavaLand2016

@oldJavaGuy#JavaLand2016 bit.ly/hackingJava
Enhancing Code at build or runtime
Sean Patrick Floyd
Hacking Java

Sean Patrick Floyd
• Search Engineer @ Zalando
• ~20 years experience
• Java, Maven, Spring, Scala, Groovy
• Twitter: @oldJavaGuy
• StackOverflow: bit.ly/spfOnSO

Scope of this talk
• Overview of non-standard techniques
• Grouped by use case
• Some techniques are more mature than
others
• Code samples and unit tests

Use Cases
• Value Objects
• Third party library patching
• Compile-time code defect analysis

Github Project
• github.com/mostlymagic/hacking-java
bit.ly/hackingJava
• Organized as Multi-Module Maven project
• Grouped by use case
• Sample code and unit tests for every
technique

Use Case: Value Objects
• Standard, well defined
behavior (equals(),
hashCode(), toString())
according to Effective Java
• Mutable or immutable, with
constructors, factory
methods or builders
• Java Boilerplate very
verbose and error-prone

The Pain
public class MyValueObject {
private final String property1; private final boolean property2;
public MyValueObject(String property1, boolean property2) {
this.property1 = property1; this.property2 = property2;
}
public String getProperty1() { return property1; }
public boolean isProperty2() { return property2; }
@Override public int hashCode() { return Objects.hash(property1, property2); }
@Override public boolean equals(Object obj) {
if (this == obj) { return true; }
else if (obj instanceof MyValueObject) {
MyValueObject other = (MyValueObject) obj;
return Objects.equals(this.property1, other.property1)
&& Objects.equals(this.property2, other.property2);
} else { return false; } }
@Override public String toString() {
return MoreObjects.toStringHelper(this)
.add("property1", property1).add("property2", property2).toString();
}
}

For comparison: Scala
case class MyValueObject(property1: String, property2: Boolean) {}
Source: http://onlyinamericablogging.blogspot.com/2014/11/move-along-now-nothing-to-see-here.html

Boilerplate
• equals() / hashCode() / toString() /
getters / setters / constructors /
compareTo()
• IDEs offer generation, but in a static
way
• potential bugs: adding or removing
fields
• Boilerplate (n.): newspaper [and IT]
slang for "unit of writing that can be
used over and over without change,"
1893, from a literal meaning (1840)
"metal rolled in large, flat plates for
use in making steam boilers."
Source: http://www.etymonline.com/
en.wikipedia.org/wiki/Boilerplate_(robot)

Plan
• Let’s look at technologies that let us define
value objects in a less-verbose way
• But with the full functionality (a test suite
will monitor correct working of getters,
equals, hashCode and toString)
• Different approaches: build-time, compile-
time, run-time

Testing
• Expected Data structure (mutable or
immutable):
User
- firstName (String)
- lastName (String)
- birthDate (LocalDate)
- addresses (List[Address])
Address
- street (String)
- zipCode (int)
- city (String)

Test suite for value objects
public abstract class BaseUserTest {
protected static final String FIRST_NAME = "Fred"; // etc.
@Test public void equalsAndHashCodeAreSymmetrical() {
Object user1 = createUser(); Object user2 = createUser();
assertThat(user1, is(equalTo(user2))); assertThat(user2, is(equalTo(user1)));
assertThat(user1.hashCode(), is(equalTo(user2.hashCode()))); }
@Test public void toStringIsConsistent() {
assertThat(createUser().toString(), is(equalTo(createUser().toString())));
String s = createUser().toString();
assertThat(s, containsString(FIRST_NAME)); /* etc. */ }
@Test public void compareToIsSymmetrical() {
Object l = createUser(), r = createUser();
assertThat(l, instanceOf(Comparable.class));
assertThat(r, instanceOf(Comparable.class));
assertThat(((Comparable) l).compareTo(r),
equalTo(((Comparable) r).compareTo(l))); }
}

Test suite (continued)
@Test public void propertyMapHasCorrectValues() {
Object instance = createUser();
Map<String, Object> map = getPropertyMap(instance);
assertThat(map, hasEntry("firstName", FIRST_NAME)); // etc.
}
private static Map<String, Object> getPropertyMap(Object obj) {
Map<String, Object> map = new TreeMap<>();
try { Arrays.stream(Introspector.getBeanInfo(obj.getClass(), Object.class)
.getPropertyDescriptors()).filter((it) -> it.getReadMethod() != null)
.forEach((pD) -> { Method m = propertyDescriptor.getReadMethod();
try { Object o = m.invoke(instance); map.put(pD.getName(), o);
} catch (IllegalAccessException | ... e) { throw new ISE(e); }});
} catch (IntrospectionException e) { throw new IllegalStateException(e); }
return propertyMap;
}
protected abstract Object createUser();

Alternative Test Suite
• Guava’s AbstractPackageSanityTests
( http://bit.ly/AbstractPackageSanityTests )
• Automatically runs sanity checks against top
level classes in the same package of the test
that extends AbstractPackageSanityTests.
Currently sanity checks include
NullPointerTester, EqualsTester and
SerializableTester.
• Nice, but not a perfect match for this use case

Annotation Processing
• JSR 269, pluggable annotation processing:
Separate compiler lifecycle, well suited for code
generation
• Service auto-discovery through ServiceLoader:
/META-INF/services/javax.annotation.processing.Processor
contains qualified processor names (
bit.ly/srvcLdr )
• Docs: bit.ly/annotationProcessing (Oracle
JavaDocs)

Project Lombok
• Name: Lombok is an Indonesian Island
neighboring Java (“it’s not quite Java, but
almost”)
• Project Lombok uses Annotation
Processing to extend the AST. It uses
internal compiler APIs (Javac and Eclipse)
• Advantages: Little code, lots of power, no
runtime dependencies
• Disadvantages: Relying on undocumented
internal APIs, adds code that is not
reflected in sources (inconsistent)
Source: http://bit.ly/1lOfPbC

Lombok: mutable example
@Data
private String property1;
private boolean property2;
}
• Generates getters, setters, equals, hashCode,
toString
• Additional fine-tuning annotations are available

Lombok: immutable example
@Data
private final String property1;
private final boolean property2;
}
• Generates constructor, getters, equals, hashCode,
toString
• Builder version also available

Google AutoValue
• https://github.com/google/auto/tree/master/value
• “AutoValue […] is actually a great tool for eliminating the
drudgery of writing mundane value classes in Java. It
encapsulates much of the advice in Effective Java […].
The resulting program is likely to be shorter, clearer, and
freer of bugs.” -- Joshua Bloch, author, Effective Java
• Advantages: Only public APIs used, no runtime
dependencies
• Disadvantages: Less power and flexibility, only
immutable types supported (or is that an advantage?)

AutoValue Sample code
@AutoValue // class needs to be abstract
public abstract class MyVO {
// use JavaBeans property names
// or simple field names
public abstract String getProperty1();
public abstract boolean isProperty2();
// factory method for instantiation
static MyVO create(String p1, boolean p2){
return new AutoValue_MyVO(p1, p2);
//"AutoValue_" + abstract class name
}
}

CGLib BeanGenerator
• https://github.com/cglib/cglib
• CGLib is a “high level” byte code manipulation
framework
• Widely used in production code, mostly by IOC and
ORM frameworks (Spring, Guice etc)
• BeanGenerator is a playground feature that can
create value types on the fly
• Works on Byte Code (Stack programming model):
https://en.wikipedia.org/wiki/Java_bytecode

Caveat
• BeanGenerator generates field + getter + setter (no
immutable types, no equals(), hashCode(), toString())
• To solve these issues, I am creating a dynamic proxy
around the generated class, which in turn uses a full
JavaBeans property map for all operations.
• /* This is O(scary), but seems quick enough in
practice. */
(Source: http://bit.ly/bigOScary )
• There is probably no sane use case for this

Don’t try this at home!
• There is (almost) no sane
reason for generating
value classes at runtime
• One possible scenario
would be a CMS that lets
you add new content types
at runtime and is backed
by a class-based ORM Source: http://www.richardbealblog.com/dont-try-this-at-home/

Code Generation
• Techniques for creating source code dynamically at
build time before compiling
• For those of us who like to “write code that writes code”
Source: http://xkcd.com/1629/ Source: http://www.memes.com/meme/487

Why code generation?
• Style wars: e.g. instanceof vs getClass()
• Custom requirements: Serializable, Jackson or
JPA annotations
• Good news: you can do almost anything
• Bad news: you have to do almost everything
yourself
• Most users will prefer “standard” solutions like
Lombok or AutoValue

Adding code generation to the Maven Build
• Maven has two dedicated lifecycle phases for code
generation, “generate-sources” and “generate-test-
sources”
• The easiest way to achieve code generation is to
execute a Groovy script in one of these phases,
using the Groovy Maven Plugin
(http://bit.ly/groovyMavenPlugin )
• By convention, Code generation uses the output
folder target/generated-sources/{generator-name}
• Never generate code to src folder!

JCodeModel
• https://github.com/phax/jcodemodel
• Fork of Sun’s abandoned JCodeModel
project, which did code generation for
JAXB
• Programmatic Java AST generation and
source code generation
• Friendly, understandable API

Code Generation with JCodeModel (in Groovy)
class DtoGenerator {
def codeModel = new JCodeModel()
public generate(List<Dto> dtos, File targetDir) {
dtos.each {
def c = codeModel._package(it.packageName)
._class(JMod.PUBLIC, it.className)
defineConstructor(c, it.properties)
defineGetters(c, it.properties)
defineEqualsMethod(c, it.properties)
defineHashCodeMethod(c, it.properties)
defineToStringMethod(c, it.properties)
defineCompareToMethod(c, it.compProperties)
}
targetDir.mkdirs(); codeModel.build(targetDir) }}

Constructor and Fields
class DtoGenerator { // continued
private defineConstructor(JDefinedClass c,
Map fields) {
def con = c.constructor(JMod.PUBLIC)
def body = con.body()
fields.each { e ->
def type = resolveType(e.value)
def f = c.field(PRIVATE | FINAL,
type, e.key)
def param = con.param(type, e.key)
body.assign(THIS.ref(f), param)
} } } // continued

Value Objects (what else)
• Techniques I haven’t explored yet, but might
in future:
• Template-based code generation
• More efficient byte code generation
(ByteBuddy)
• Interface-based proxies
• JEP 169 (http://openjdk.java.net/jeps/169)

Use Case: Library Patching
• Scenario: You have to use a third party library,
which has known defects
• Best choice: file an issue and / or submit a Pull
Request to the library owner
• I’ll assume this didn’t work, you are dependent on
the library, but you don’t want to fork it, since you
want to stay current
• Techniques that will let you patch lib in an
automated way (ideally in a CI / CD pipeline)

Options
• If you have access to the source code, use a
source code parser and dynamically patch the
sources
• Otherwise, manipulate byte code (change or
intercept existing code). Can be done statically
at build time or dynamically at class load time
• Create a static patch and write tests which
make sure the patch worked (I won’t explore
this option)

Baseline
• As example I have written a trivial example class,
with some pretty stupid, but harmless bugs:
public class FicticiousExample {
public Integer yUNoReuseInteger(final int value) {
System.setProperty(RAN_BUGGY_CODE, TRUE);
return new Integer(value); }
public String yUStringConcatInLoop(Iterable<String> data,
String delim) {
System.setProperty(RAN_BUGGY_CODE, TRUE);
String v = "";
final Iterator<String> it = data.iterator();
if (it.hasNext()) { v += it.next(); }
while (it.hasNext()) { v += delim + it.next(); }
return value;
}
}

Test suite
• In the test, I call the methods and assert that they work
correctly and that the system properties weren’t set
public abstract class FicticiousExamplePatchTest {
private FicticiousExample fe;
@After public void checkForEvilSystemProperty() {
assertNull(System.getProperty(RAN_BUGGY_CODE)); }
@Before public void initObject() {
fe = new FicticiousExample(); }
@Test public void assertCorrectIntegerBehavior() {
assertTrue(fe.yUNoReuseInteger(123)
== fe.yUNoReuseInteger(123));
assertFalse(fe.yUNoReuseInteger(1234)
== fe.yUNoReuseInteger(1234));
}
@Test public void assertCorrectStringBehavior() { // etc.

AspectJ
• https://eclipse.org/aspectj/
• Aspect-oriented language
(.aj format or Java with @AspectJ
annotations)
• ajc = AspectJ Compiler (after javac,
instead of javac)
• static compilation or load time weaving
through agent
• Docs are ancient / non-existent, use
mailing list or read “the” book:
bit.ly/ajInAction

Patching Aspect
public aspect FicticiousExamplePatch{
public pointcut integerMethodCalled(int value) :
execution(* FE.yUNoReuseInteger(..)) && args(value);
public pointcut stringMethodCalled(Iterable<String> it, String s):
execution(* FE.yUStringConcatInLoop(..)) && args(it, s);
Integer around(int i) : integerMethodCalled(i){
return Integer.valueOf(i); }
String around(Iterable<String> it, String s) :
stringMethodCalled(it, s){
java.util.Iterator<String> iterator = it.iterator();
StringBuilder sb = new StringBuilder();
if(iterator.hasNext()){
sb.append(iterator.next());
while(iterator.hasNext()){
sb.append(s).append(iterator.next()); }
}
return sb.toString(); }
}

AspectJ capabilities
• Extending / replacing / enriching code
• Inter-type declarations (~= Traits)
• Policy Enforcement (define custom compiler
errors)
• Very flexible, can work on source or byte code
• Standard usage: Cross-cutting concerns
(security, logging etc.)

Source Code Manipulation
• Extract sources to generated-sources
directory
• Parse and manipulate the sources (non-
idempotent!?)
• Compile sources and distribute your library

JavaParser
• http://javaparser.github.io/javaparser/
• Parsers generated from JavaCC (Java Compiler
Compiler) -> complete Java 1.8 grammar support
• API not as friendly as JCodeModel (no management
of types and references etc.), but the only available
fully functional Java source code parser
• Again, I’ll be using Groovy to do the actual work. It
could be done in Java too, but the build setup would
be more complex

PatchVisitor
class MyVisitor extends VoidVisitorAdapter<Void> {
public void visit(MethodDeclaration n,
Object arg) {
if (n.name == "yUStringConcatInLoop") {
n.body = new BlockStmt() // delete body
patchStringMethod(n.body, n.parameters[0],
n.parameters[1])
} else if (n.name == "yUNoReuseInteger") {
n.body = new BlockStmt() // delete body
patchIntegerMethod(n.body, n.parameters[0])
} else super.visit(n, arg) }

PatchVisitor (Integer method)
private patchIntegerMethod(
BlockStmt b, Parameter p) {
def t = new ClassOrInterfaceType("Integer");
def e = new TypeExpr(); e.type = t
def mc = new MethodCallExpr(e, "valueOf")
mc.args.add(new NameExpr(p.id.name))
b.getStmts().add(new ReturnStmt(mc))
}
// all this for
// return Integer.valueOf(i);

Tradeoff
• AST manipulation is hard,
very verbose and error prone
• But: It’s still better than trying
to do it with Regex (insert
obligatory Regex Cthulhu link
here)
• friendlier Alternative:
walkmod.com (built on
JavaParser, “walkmod is an
open source tool to apply and
share your own code
conventions.”)
Source: http://subgenius.wikia.com/wiki/Cthulhu

Use Case: Defect Analysis
• Identify bug patterns,
reject them at compile time
• Not mentioning the “good
old” tools: CheckStyle,
PMD, FindBugs. They are
best used in a separate CI
build
• Focus on tools that hook
directly into the compile
process
Source: http://sequart.org/magazine/59883/cult-classics-starship-troopers/

Test Harness
public abstract class AbstractDefectDetectionTest extends AbstractCompilerTest {
private DefectAnalysisEngine engine;
@Before public void setupEngine() { this.engine = instantiateEngine(); }
@Test public void detectWellBehavedClass() {
CompilationResult r = engine.compile(sourceFileFor(wellBehavedClass()));
assertThat(r, isSuccess());
}
@Test public void detectIllBehavedClass() {
CompilationResult r = engine.compile(sourceFileFor(illBehavedClass()));
assertThat(r, isFailureWithExpectedMessage(expectedErrorMessage()));
}
protected abstract DefectAnalysisEngine instantiateEngine();
protected abstract Class<? extends WellBehaved> wellBehavedClass();
protected abstract Class<? extends IllBehaved> illBehavedClass();
protected abstract String expectedErrorMessage();
}

ForkedRun
• Helper class that calls a Java class in a
separate process, copying the Classpath
from the local context, abstracting the output
away to a CompilationResult class, that we
can run matchers against.
• Fluent API with matchers for converting
classpath to bootclasspath and other nice
features
• http://bit.ly/forkedRun

ForkedRun (sample)
public final CompilationResult run() {
final Set<String> classPath = new LinkedHashSet<>();
final Set<String> bootPath = new LinkedHashSet<>();
try {
dispatchClassPath(classPathElements, bootPathElements);
String javaExe = getExecutable();
List<String> cmds =
buildCommands(classPath, bootPath, javaExe);
ProcessBuilder pb = new ProcessBuilder().command(cmds);
Process proc = pb.start();
List<String> errorMsg = gatherOutput(proc);
int status = proc.waitFor();
return new CompilationResult(status == 0, errorMsg);
} catch (InterruptedException | … | URISyntaxException e) {
throw new IllegalStateException(e); } }

False positives
• Example: Immutability (impossible to reliably
detect, by any technology known to me)
public final class ImmutableUser{
private final String name; private final Date birthDate;
private final List<String> nickNames;
public ImmutableUser(String name, List<String> nickNames,
Date birthDate) {
this.name = name;
this.nickNames = ImmutableList.copyOf(nickNames);
this.birthDate = new Date(birthDate.getTime()); }
public String getName() { return name; }
public List<String> getNickNames() {
return ImmutableList.copyOf(nickNames); }
public Date getBirthDate() { return new Date(birthDate.getTime()); }
}

What can we detect?
• Forbidden classes, erroneous usages
• Package-level architecture restrictions
(MicroServices or OSGI are a better
solution)
• Implementation inconsistent with
annotations or interface (e.g. Nullness)

Google Error Prone
• http://errorprone.info/
• Google 20% project
• Wrapper around javac, with compatible API
• Many defined bug-patterns, most specific to
Google (Android, Protobuf, Gwt, Guice)
• New Bug Patterns can only be added via Pull
Request to Google :-(
• Integrated with Maven, Gradle, Ant etc.

Obscure bug patterns
• Example: Regex bug detection
public class IllBehavedRegex {
static List<String> splitByAlpha(String s) {
return Arrays.asList(
s.split("[a-z")); // bad pattern
}
}

Checker Framework
• http://types.cs.washington.edu/checker-framework/
• The Checker Framework enhances Java’s type system to
make it more powerful and useful. This lets software
developers detect and prevent errors in their Java
programs. The Checker Framework includes compiler plug-
ins ("checkers") that find bugs or verify their absence. It
also permits you to write your own compiler plug-ins.
• Disadvantage: needs to be installed separately, hard to
integrate in a build system
• But: checkers can be used as plain annotation processors

Java 8 Type Annotations
• As of the Java SE 8
release, annotations can
also be applied to any type
use […] A few examples of
where types are used are
class instance creation
expressions (new), casts,
implements clauses, and
throws clauses.
http://bit.ly/typeAnnotations
• The Checker Framework
embraces these new
annotation usages
Source: http://memegenerator.net/

Example: Nullness
public class WellbehavedNullness {
@PolyNull String nullInOut(@PolyNull String s) {
if (s == null) return s;
return s.toUpperCase().trim();
}
@MonotonicNonNull
private String initallyNull;
public void assignField(@NonNull String value) {
initallyNull = checkNotNull(value);
}
@Nonnull public String getValue() {
if (initallyNull == null)
initallyNull = "wasNull";
return initallyNull;
} }
Source: https://imgflip.com/memegenerator

AspectJ (again)
• AspectJ allows custom compiler
errors, according to static
checks
• Architecture checks: package a
may not access package b
• Forbidden usage patterns: e.g.
Spring MVC controller may not
take OutputStream param
• Forbidden classes: Enforce
deprecation of legacy classes
• Most of these problems can be
solved differently
(MicroServices etc.)
Source: http://www.someecards.com/usercards/viewcard/MjAxMy1jZWRhODllNTI3YWI4Yjkz

Forbid usage of deprecated types
public aspect PolicyEnforcementAspect{
pointcut badCall() :
call(* Hashtable.*(..))
|| call(Hashtable.new(..))
|| call(* Vector.*(..))
|| call(Vector.new(..));
declare error : badCall()
"Hashtable and Vector are deprecated!";
}

Overview of discussed techniques
• Obviously, this is an opinion, not advice. Use at your own risk.
• Author is not affiliated with any of the above libraries.
Technique Have I used in real applications? Do I recommend technique?
Lombok yes maybe
AutoValue yes yes
CGLib BeanGenerator no no
JCodeModel yes yes
AspectJ (patching) yes maybe
JavaParser yes maybe
ErrorProne no maybe
Checker Framework no yes
AspectJ (policy enforcement) yes yes

Where to go from here
• Look at the github code, send me a PR if
you have improvements
• Contact me if you have questions
• Suggest more techniques / use cases

Source: http://www.gapingvoidart.com/gallery/any-questions/

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