Power tools in Java

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Power Tools in Java SDK
Elek Márton
DPC Consulting Kft

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• Modular static html site generator
• (like Jekyll, github pages)
• Do custom transformation on each text file
• Example transformations:
• ascii link break → <br/>
• plain text → text with html header/footer
<html>....<body>{...}</body></html>
Example application

3
Code example – types
public class Resource {
private Map<String, String> headers = new HashMap<>();
private String content;
…
}
public interface Transformer {
public void transform(Resource resource);
}

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Code example – processing
System.out.println("Transforming " + file);
Resource r = new Resource(new String(Files.readAllBytes(file)));
for (Transformer transformations : transformers) {
transformations.transform(r);
}
return r;

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• Kezdeti megoldás:
private List<Transformer> transformations = new ArrayList();
public Generator() {
transformations.add(new HtmlFrameTransformator());
transformations.add(new Nl2BrTransfomer());
}
for (Transformer transformations : transformers) {
transformations.transform(r);
}
• Cél
• Dinamikusan kiterjeszthető
• lib/-be új JAR → automatikusan elinduljon
Modularitás

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Step one: using descriptors
• bin/
• site-generator.bat
• site-generator.sh
• lib/
• site-generator-1.0.jar
• site-generator-extension-1.0.jar
• config
• transform-implementations.cfg:
hu.dpc.javabar.powertools.impl.HtmlFrameTransformator
hu.dpc.javabar.powertools.impl.Nl2BrTransfomer
hu.dpc.javabar.powertools.impl.YoutubeTransformer

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Step two: descriptor on
classpath
• bin/
• site-generator.bat, site-generator.sh
• lib/
• site-generator-1.0.jar
• site-generator-1.0.jar:/transform-implementations.cfg
hu.dpc.javabar.powertools.impl.HtmlFrameTransformator
hu.dpc.javabar.powertools.impl.Nl2BrTransfomer
• site-generator-extension-1.0.jar
• site-generator-extension-1.0.jar:/transform-implementations.cfg
hu.dpc.javabar.powertools.impl.YoutubeTransformer
Enumeration<URL> resources =
getClass().getClassLoader().getResources("/transform-implementations.cfg");

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Step three:
Service Provider Interface
• Standard, part of the JDK
• Descriptor are on the classpath
• Contains the class names of the implementations
• Using nameconvention (META-INF/services/hu.dpc...Interface)
• Simple API (ServiceLoader), only a few classes
• Not included:
• Dependency management
• Versioning
• Priority handling
• Only answers the question:
• Who implemented a specific interface

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SPI code example
• META-INF/services/hu.dpc.javabar.powertools.api.Transformer
• hu.dpc.javabar.powertools.impl.HtmlFrameTransformer
• hu.dpc.javabar.powertools.impl.Nl2BrTransfomer
ServiceLoader<Transformer> transformers =
ServiceLoader.load(Transformer.class);
for (Transformer transfomer : transformers) {
...
}

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SPI – where is it used
• (almost) everywhere
• multiple location at JDK (eg. XML processor
implementation)
• Even the OSGi has adapter for SPI
• Very effective even if it's very simple
• Easy solution to avoid circular dependencies
of the projects

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How to generate
SPI descriptors?
• Service Provider Interface
META-INF/services/...
should be created and filled with
implementation automatically

14
Annotation processors
• Part of the javac from Java 6
• Run at compile time
• Class/descriptor/... files could be generated
based on specific annotated classes
• Could be activated with SPI
• META-INF/services/
javax.annotation.processing.Processor
• Multiple iteration (processing the generated files)
• Parameters could be used

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Annotation processor
public @interface Service {
}
@Service
public class Nl2BrTransfomer implements Transformer {
@Override
public void transform(Resource resource) {
resource.setContent(
resource.getContent().replaceAll("nn", "<br/>")
);
}
}

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Annotation processor example
@SupportedAnnotationTypes("hu.dpc.javabar.powertools.Service")
@SupportedSourceVersion(SourceVersion.RELEASE_7)
public class ServiceAnnotationProcessor extends AbstractProcessor {
Map<String, Set<String>> services = new HashMap<String, Set<String>>();
@Override
public boolean process(Set<? extends TypeElement> annotations,
RoundEnvironment roundEnv) {
…
}
}

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Map<String, Set<String>> services = new HashMap<String, Set<String>>();
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, "@service generation");
if (roundEnv.processingOver()) {
writeOutServices(roundEnv);
return false;
}
//process annotations
...
}

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public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
//write out the file
...
Elements elements = processingEnv.getElementUtils();
for (Element element : roundEnv.getElementsAnnotatedWith(Service.class)) {
TypeElement type = (TypeElement) element;
TypeElement contract = (TypeElement) ((DeclaredType) type.getInterfaces().get(0)).asElement();
String cn = elements.getBinaryName(contract).toString();
Set<String> v = services.get(cn);
if (v == null) {
services.put(cn, v = new TreeSet<String>());
}
v.add(cn);
}
return false;
}

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Generating code
for (Element element : roundEnv.getElementsAnnotatedWith(Service.class)) {
TypeElement type = (TypeElement) element;
JavaFileObject jfo = processingEnv.getFiler().createSourceFile(
type.getQualifiedName() + "GeneratedInfo");
BufferedWriter bw = new BufferedWriter(jfo.openWriter());
bw.append("package ");
bw.append(((PackageElement) type.getEnclosingElement()).getQualifiedName());
bw.append(";n");
bw.append("import hu.dpc.javabar.powertools.Service;n");
bw.newLine();
bw.newLine();
bw.append("@Service n");
bw.append(String.format("public class %s{", type.getSimpleName()));
bw.newLine();
bw.append("public static final boolean PROCESSED = true;" );
bw.append("}");
bw.flush();

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Project to generate SPI
• To generate SPI descriptors, try
• http://metainf-services.kohsuke.org/
• https://code.google.com/p/spi/
<dependency>
<groupId>org.kohsuke.metainf-services</groupId>
<artifactId>metainf-services</artifactId>
<version>1.1</version>
<optional>true</optional>
</dependency>

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Documentation of SPI
implementation
/**
* Replace new double new lines with html line break.
*/
@Service("Replace new double new lines with html line break")
public class Nl2BrTransfomer implements Transformer {
@Override
public void transform(Resource resource) {
resource.setContent(resource.getContent().replaceAll("nn", "<br/>"));
}
}
Available transformations:
HtmlFrameTransformator : ...
Nl2BrTransfomer : Replace new double new lines with html line break

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How to access javadoc at
runtime?
>>> def hello(name):
... """Print hello for the name"""
... print("Hello", name)
...
>>> hello("Bela")
Hello Bela
>>> print(hello.__doc__)
Print hello for the name
user=> (defn hello "Print hello for
the name" [name] (str "Hello " name))
#'user/hello
user=> (hello "Bela")
"Hello Bela"
user=> (get (meta (var hello)) :doc)
"Print hello for the name"
• The documentation of the annotated service
class should come from javadoc documents
• Easy with other languages(Python, Clojure)

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Javadoc – Doclets
• Formatting classes for custom javadoc
generation
• Doclet API could access all of the javadoc and
class metadata information
• Could be used to generate custom javadoc
page but also to generate XML descriptors

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Doclet example
public class GenerateDocletResource {
public static boolean start(RootDoc rootDoc) {
for (ClassDoc cd : rootDoc.specifiedClasses()) {
Path p = Paths.get(root + "/.../" +
cd.qualifiedTypeName() + ".javadoc");
Files.createDirectories(p.getParent());
Files.write(p, cd.commentText().getBytes());
}
return true;
}
}

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Doclet loading
public String getJavadoc(Class type) {
String resourceName = "/" + type.getCanonicalName() + ".javadoc";
try (InputStream s = getClass().getResourceAsStream(resourceName)){
if (s == null) {
return "Javadoc resource is missing for class " + type;
}
return new Scanner(s).useDelimiter("//Z").next();
} catch (IOException ex) {
...
}
}

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Using it from Gradle
dependencies {
doclet project(":javadoc-metadata-generator")
}
task generateDocs(type: Javadoc) {
source = sourceSets.main.allJava
options = new SimpleJavadocOption()
options.docletpath = configurations.doclet.files.asType(List)
options.doclet = "hu.dpc.javabar.powertools.doc.GenerateDocletResource"
}
compileJava.dependsOn generateDocs

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Using it from Maven
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-javadoc-plugin</artifactId>
<version>2.9.1</version>
<executions>
<execution>
<id>doc-resource-generator</id>
<phase>prepare-package</phase>
<goals>javadoc</goals>
</execution>
</executions>
<configuration>
<doclet>org.umlgraph.doclet.UmlGraphDoc</doclet>

<docletArtifact>
<groupId>org.umlgraph</groupId>
<artifactId>doclet</artifactId>
<version>5.1</version>
</docletArtifact>
<additionalparam>-views</additionalparam>
<useStandardDocletOptions>true</useStandardDocletOptions>
</configuration>
</plugin>

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Java Management Extension
(JMX)
• Well known solution from the Standard Java
API
• Management
• Monitoring
• All of the JVM information could be accessed
as well
• Custom metrics could be added

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JMX JSR
• JSR 3: Java Management Extension (JMX)
• JSR 160: JMX Remote API
• JSR 255: JMX 2.0 = JSR 3 + 160 + …
(Inactive)
• JSR 262: Web Services Connector for JMX
(Inactive)
• Jolokia: REST interface adapter

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JMX Architecture
Source wikipedia

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JMX – Probe level
• MBean – Managed Object: bármilyen
resource
• Attributes
• Operations
• Notifications
• MBean types
• Standard
• Dynamic
• MXBeans

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Simple JMX example
public interface GeneratorMBean {
public void stop();
public int getProcessedFileNo();
}
MBeanServer server =
ManagementFactory.getPlatformMBeanServer();
ObjectName name =
new ObjectName("hu.dpc.javabar.powertools", "name", "Generator");
server.registerMBean(generator, name);
//generator implements GeneratorMBean

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JMX Architecture
Source wikipedia

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JMX platform Mbean server

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JMX custom Mbean server
MBeanServer server = MbeanServerFactory.createMBeanServer();
Registry registry = LocateRegistry.createRegistry(1234);
JMXServiceURL jmxServiceURL = new JMXServiceURL("rmi",
"localhost",
1235,
"/jndi/rmi://localhost:1234/jmx");
JMXConnectorServer connectorServer =
JMXConnectorServerFactory.newJMXConnectorServer(jmxServiceURL, null, server);
connectorServer.start();
System.out.println(jmxServiceURL);
// service:jmx:rmi://localhost:1235/jndi/rmi://localhost:1234/jmx

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JMX client API
JMXServiceURL url =
new JMXServiceURL("service:jmx:rmi://localhost:1235/jndi/rmi://localhost:1234/jmx");
JMXConnector jmxc = JMXConnectorFactory.connect(url, null);
MBeanServerConnection mbsc = jmxc.getMBeanServerConnection();
ObjectName name = new ObjectName("hu.dpc.javabar.powertools", "name", "Generator");
GeneratorMBean mbean = JMX.newMBeanProxy(mbsc, name, GeneratorMBean.class);
System.out.println(mbean.getProcessedFileNo());
//12

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JMX Architecture
Source wikipedia

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JMX custom MBeanServer
MBeanServer server = MbeanServerFactory.createMBeanServer();
Registry registry = LocateRegistry.createRegistry(1234);
JMXServiceURL jmxServiceURL = new JMXServiceURL("rmi",
"localhost",
1235,
"/jndi/rmi://localhost:1234/jmx");
JMXConnectorServer connectorServer =
JMXConnectorServerFactory.newJMXConnectorServer(jmxServiceURL, null, server);
connectorServer.start();
System.out.println(jmxServiceURL);
// service:jmx:rmi://localhost:1235/jndi/rmi://localhost:1234/jmx

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JMX platform MBeanServer

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Java Agents
• Written in Java or C/C++
• Native: Java Virtual Machine Tool Interface (JVM TI)
• -agentlib, -agentpath
• Java:
• -javaagent
• Features
• run custom command (in the application JVM-ben)
• manipulating bytecode loading
• Activating
• Before application startup (-javaagent)
• Attaching at runtime (Attach API)

44
Java Agent interface
public static void premain(String agentArgs, Instrumentation inst) {
}
• Intstrumentation
• addTransformer(ClassFileTransformer transformer)
• Registers the supplied transformer.
• appendToBootstrapClassLoaderSearch(JarFile jarfile)
• Specifies a JAR file with instrumentation classes to be defined by the bootstrap class
loader.
• appendToSystemClassLoaderSearch(JarFile jarfile)
• Specifies a JAR file with instrumentation classes to be defined by the system class
loader
• ClassFileTransformer.
• transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer)

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Activating Java Agent
• Before application start
• -javaagent:<jarpath>[=<options>]
• META-INF/MANIFEST.MF: Premain-Class: …
• public static void premain(...)
• During application run
• Attach API
• META-INF/MANIFEST.MF: Agent-Class: …
• public static void agentmain(...)

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Attach API
//JVMs of the CURRENT user
List<VirtualMachineDescriptor> vms = VirtualMachine.list();
for (VirtualMachineDescriptor virtualMachineDescriptor : vms) {
VirtualMachine vm =
VirtualMachine.attach(virtualMachineDescriptor);
if (vm != null) {
System.out.println(virtualMachineDescriptor.displayName());
System.out.println("Java version = " +
vm.getSystemProperties().
getProperty("java.version"));
}
}

47
JMX connection – with agents
static final String CONNECTOR_ADDRESS =
"com.sun.management.jmxremote.localConnectorAddress";
VirtualMachine vm = VirtualMachine.attach(id);
String connectorAddress = vm.getAgentProperties().getProperty(CONNECTOR_ADDRESS);
if (connectorAddress == null) {
String agent = vm.getSystemProperties().getProperty("java.home") +
File.separator + "lib" + File.separator + "management-agent.jar";
vm.loadAgent(agent);
connectorAddress = vm.getAgentProperties().getProperty(CONNECTOR_ADDRESS);
}
JMXServiceURL url = new JMXServiceURL(connectorAddress);
JMXConnector = JMXConnectorFactory.connect(url);

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Where is it used?
• Bytecode manipulation
• AspectJ
• Spring
• Persistence frameworks
• BTrace
• premain/agent:
• jre/lib/management-agent.jar

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Summary
• JDK Power tools
• Service Provider Interface – for modularity
• Annotation processors – compile time
processing
• Javadoc doclet – Javadoc (pre)processing
• JMX – runtime metrics, methods
• Agentek, Attach API – byte code manipulation,
runtime modification

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Credits
• Thanks for the image authors:
• Container, Sandor Volenszki, Creative Commons
• Blueprint, Will Scullin, Creative Commons
• Robot, Avram Cheaney, Creative Commons
• Control Panel, Paul, Creative Commons
• PCB, Karl-Ludwig G. Poggemann, Creative
Commons

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Elek Márton / DPC Consulting
marton.elek@dpc.hu
http://www.meetup.com/bpjavabar/

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