Archive for the ‘en-US’ Category

I wrote the following notice on Sidney COHEN‘s profile on LinkedIn:

Sidney has a deep knowledge of the technologies he uses, such as Tibco systems. In several occasions, he was able to liaise with the key people of the right teams, perform the good actions and deliver the best solutions to various issues. Moreover, he has learnt complex Java technologies in a short time.
I recommend Sidney as a good worker and a friendly team mate

 

I wrote the following notice on Yann BLAZART‘s profile on LinkedIn:

Yann has a wide knowledge in technologies, specifications and implementations of JEE. He has shown his ability to propose and implement technical solutions to various technical and functional issues and subjects. Besides, by helping his colleagues, he succeeded in raising the general level of the team. All this is why I recommend Yann as a Java developer and architect.

The blockchain emerged as a revolutionary technology, capturing significant attention with its potential to reshape industries and redefine trust. At Devoxx France 2016, Benoît Lafontaine and Yann Rouillard delivered a comprehensive university session delving into the intricacies of this much-hyped technology, moving beyond the buzzwords to explore its technical underpinnings, evolutions, and practical implications. Their detailed exposition covered the foundational principles of Bitcoin, the expanded capabilities introduced by platforms like Ethereum, the concept and implementation of smart contracts, various use cases, and the broader societal questions raised by distributed ledger technologies.

Demystifying the Blockchain: The Foundation of Bitcoin

To truly grasp the essence of blockchain, one must first understand its initial and most prominent implementation: Bitcoin. More than just a digital currency, Bitcoin introduced a novel distributed ledger technology that enables secure, transparent, and tamper-resistant record-keeping without relying on a central authority. The core of Bitcoin’s technical functioning lies in its chain of blocks. Each block contains a list of verified transactions, a timestamp, and a reference (cryptographic hash) to the preceding block, creating an immutable historical record.

The lifecycle of a Bitcoin transaction begins when a user initiates a transfer of value. This transaction is broadcast to the Bitcoin network. Nodes on the network validate the transaction based on a set of rules, ensuring the sender has sufficient funds and the transaction is correctly formatted. Once validated, the transaction is added to a pool of unconfirmed transactions.

The process of adding new blocks to the chain is handled by miners through a mechanism called Proof-of-Work (PoW). Miners compete to solve a complex computational puzzle, which essentially involves finding a number (a “nonce”) such that when added to the block data and hashed, the resulting hash meets certain criteria (e.g., starts with a specific number of zeros). This hashing process is computationally intensive but easy to verify. The first miner to find a valid nonce and create a new block broadcasts it to the network. Other nodes verify the block’s validity, including the PoW, and if correct, add it to their copy of the blockchain. This block then becomes the latest link in the chain.

Cryptography plays a vital role in ensuring the security and integrity of Bitcoin. Hashing algorithms (like SHA-256 used in Bitcoin) produce a unique fixed-size string (the hash) from input data. Even a minor change in the input data results in a completely different hash. This property is used to link blocks and verify data integrity. Digital signatures, based on public-key cryptography, are used to authorize transactions. Each user has a pair of keys: a private key (kept secret) used to sign a transaction and a public key (shared freely) used by others to verify the signature, ensuring that only the owner of the funds can authorize their transfer.

One of the fundamental challenges in a distributed system like Bitcoin is reaching consensus among participants on the correct state of the ledger, especially in the presence of potentially malicious actors. Proof-of-Work is Bitcoin’s consensus mechanism. By requiring significant computational effort to create a new block, it makes it economically infeasible for a malicious party to alter past transactions. To successfully tamper with a block, an attacker would need to redo the PoW for that block and all subsequent blocks faster than the rest of the network combined. This leads to the concept of the 51% attack, where if an entity controls more than 51% of the network’s total mining power, they could potentially manipulate transactions. However, the sheer scale of the Bitcoin network’s hashing power makes achieving a 51% attack incredibly difficult and prohibitively expensive.

Beyond Bitcoin: Exploring Ethereum and Smart Contracts

While Bitcoin demonstrated the power of a decentralized ledger for peer-to-peer currency transactions, its scripting language is intentionally limited, primarily designed for simple payment logic. The next wave of blockchain innovation arrived with platforms like Ethereum, which expanded the potential of blockchain technology far beyond just cryptocurrencies. Ethereum introduced the concept of a decentralized world computer capable of executing code, paving the way for a wide range of decentralized applications.

Ethereum’s core innovation is the Ethereum Virtual Machine (EVM), a Turing-complete virtual machine that can execute code deployed on the Ethereum blockchain. This code comes in the form of smart contracts. A smart contract is essentially a program stored on the blockchain that automatically executes predefined actions when specific conditions are met. Unlike traditional contracts, which are interpreted and enforced by legal systems, smart contracts are self-executing and enforced by the code itself, running on the decentralized and immutable ledger.

The primary language for writing smart contracts on Ethereum is Solidity, a high-level, contract-oriented language. Solidity’s syntax is influenced by languages like C++, Python, and JavaScript. Benoît and Yann provided examples of Solidity code, illustrating how to define state variables, functions, and events within a contract. These contracts can represent anything from simple tokens and voting mechanisms to complex financial agreements and decentralized autonomous organizations.

Deploying a smart contract involves compiling the Solidity code into EVM bytecode and then sending a transaction to the Ethereum network containing this bytecode. Once deployed, the contract resides at a specific address on the blockchain and its functions can be invoked by other users or contracts through transactions. The execution of smart contract functions requires gas, a unit of computation on the Ethereum network, paid for in Ether (ETH), Ethereum’s native cryptocurrency. This gas mechanism incentivizes efficient code and prevents denial-of-service attacks. The ability to write and deploy arbitrary code on a decentralized and immutable ledger opened up a vast landscape of possibilities for creating decentralized applications (DApps).

Alternative Consensus Mechanisms and Decentralized Autonomous Organizations

While Proof-of-Work (PoW) has proven effective for Bitcoin’s security, its energy consumption is a significant concern. This has led to research and development into alternative consensus mechanisms. Proof-of-Stake (PoS) is a prominent alternative where the right to create new blocks and validate transactions is determined by the amount of cryptocurrency a validator holds and is willing to “stake” as collateral. In PoS, validators are chosen to create blocks based on their stake size and other factors, rather than computational power. If a validator attempts to validate fraudulent transactions, they risk losing their staked amount. PoS is generally considered more energy-efficient than PoW. At the time of the talk, Ethereum was planning a transition from PoW to a PoS consensus mechanism called Casper, a transition that has since been completed.

The capabilities of smart contracts extend to enabling entirely new forms of organization. A Decentralized Autonomous Organization (DAO) is an organization whose rules and decision-making processes are encoded directly into smart contracts on a blockchain. Once deployed, a DAO operates autonomously according to its pre-programmed rules, without the need for central management. Funding, governance, and operations are all managed through the smart contracts and the participation of token holders who vote on proposals. The talk touched upon the concept of DAOs and their potential to create more transparent and democratic organizational structures. However, the early history of DAOs also includes cautionary tales, such as “The DAO” hack in 2016, which highlighted the critical importance of rigorous security auditing for smart contracts managing significant assets.

Real-World Applications and Societal Impact

The potential applications of blockchain technology span numerous industries beyond finance. Benoît and Yann explored various use cases that were being studied or already implemented in 2016. In finance, beyond cryptocurrencies, blockchain can streamline cross-border payments, facilitate peer-to-peer lending, and improve trade finance. In supply chain management, it can provide transparent and verifiable tracking of goods from origin to destination. For identity management, blockchain could enable self-sovereign identity solutions, giving individuals more control over their personal data. Other potential applications discussed included decentralized marketplaces, intellectual property management, voting systems, and even decentralized energy grids.

The advantages offered by blockchain technology, such as transparency (for public blockchains), immutability, security through cryptography, and disintermediation (removing the need for central authorities), make it attractive for scenarios where trust and verification are paramount. However, challenges remain, including scalability limitations of some blockchains, regulatory uncertainty, the difficulty of correcting errors on an immutable ledger, and the complexity of developing and securing smart contracts.

Beyond the technical and practical applications, blockchain introduces profound social implications. It challenges existing power structures by enabling decentralization and disintermediation. It raises questions about governance in decentralized networks, the legal status of smart contracts, and the impact on privacy in a transparent ledger. The technology empowers individuals with greater control over their assets and data but also requires a higher degree of individual responsibility. The discussion during the session underscored that blockchain is not just a technical innovation but also a socio-technical one with the potential to reshape how we organize and interact in the digital age.

In conclusion, the Devoxx France 2016 session on the blockchain provided a timely and detailed exploration of this burgeoning technology. By dissecting the mechanics of Bitcoin, presenting the advancements brought by Ethereum and smart contracts, discussing alternative consensus models and DAOs, and examining a variety of use cases and societal impacts, Benoît Lafontaine and Yann Rouillard offered attendees a clearer understanding of what lay beyond the hype and why this technology warranted serious attention from developers and businesses alike. The session emphasized that while challenges existed, the potential for blockchain to drive innovation across numerous sectors was undeniable.

Links:

• Benoît Lafontaine’s Blog Post on OCTO Talks!
• OCTO Technologies official website
• Bitcoin official website
• Ethereum official website
• Solidity official website
• The DAO hack explanation

Hashtags: #Blockchain #Bitcoin #Ethereum #SmartContracts #ProofOfWork #ProofOfStake #DAO #Cryptocurrency #Decentralization #DistributedLedger #FinTech #Web3 #Solidity #EVM #OCTOTechnologies #BenoitLafontaine #YannRouillard #DevoxxFR

Abstract

I am glad and proud to announce the publication of “Android Application Development with Maven”, on March 15th 2015, by Packt.

Direct link: https://www.packtpub.com/apache-maven-dependency-management/book

Alternate locations: Amazon.com, Amazon.co.uk, Barnes & Noble.

On this occasion, I’d like to thank all Packt team for allowing me achieving this project.

What you will learn from this book

• Integrate Maven with your favorite Android IDE
• Install and configure Maven with your local development environment
• Create the proper Maven structure for both standalone Android applications or applications that are part of a bigger project
• Run unit tests using popular frameworks such as Robolectric and collect coverage information using Maven plugins
• Configure a variety of different tools such as Robotium, Spoon, and Selendroid to run integration tests
• Handle dependencies and different versions of the same application
• Manage and automate the release process of your application inside/outside Google Play
• Discover new tools such as Eclipse, IntelliJ IDEA/Android Studio, and NetBeans, which perfectly integrate with Maven and boost your productivity

In Detail

Android is an open source operating system used for smartphones and tablet computers. The Android market is one of the biggest and fastest growing platforms for application developers, with over a million apps uploaded every day.

Right from the beginning, this book will cover how to set up your Maven development environment and integrate it with your favorite IDE. By sequentially working through the steps in each chapter, you will quickly master the plugins you need for every phase of the Android development process. You will learn how to use Maven to manage and build your project and dependencies, automate your Android application testing plans, and develop and maintain several versions of your application in parallel. Most significantly, you will learn how to integrate your project into a complete factory.

Approach

Learn how to use and configure Maven to support all phases of the development of an Android application

Who this book is for

Android Application Development with Maven is intended for Android developers or devops engineers who want to use Maven to effectively develop quality Android applications. It would be helpful, but not necessary, if you have some previous experience with Maven.

Table of content

• 1: Beginning with the Basics
• 2: Starting the Development Phase
• 3: Unit Testing
• 4: Integration Testing
• 5: Android Flavors
• 6: Release Life Cycle and Continuous Integration
• 7: Other Tools and Plugins

Abstract

On migrating an application deployed through Java WebStart from JRE 6 to JRE 7 (1.7.0_71 ; the problem is present since 1.7.0_4X), system properties are ignored.
As a reminder, system properties are declared in JNLP file as:

[xml]<property name="foo" value="boo"/>[/xml]

They are equivalent to execute JRE with -D arguments, such as -Dfoo=boo for the XML block above.

Launching the JNLP file with javaws file.jnlp -Dfoo=boo or even javaws file.jnlp -J-Dfoo=boo does not fix the issue.

Quick fix

Rename all system properties, so that they are prefixed with jnlp..
Eg, replace:
[xml]<property name="foo" value="boo"/>[/xml]
with:
[xml]<property name="jnlp.foo" value="boo"/>[/xml]
Do not forget to update any part of the code calling these properties!

For a sadistic reason I still ignore, properties that are not prefixed with jnlp. are no more considered by Java WebStart included in JRE 7.

Case

My application consists in a WAR I deploy on Tomcat or Jetty during the development phase. I execute Eclipse Jetty either in standalone, or via Maven Jetty plugin – most of the time.

On updating and deploying the application on my laptop (which is not my primary development machine), I get the following error with Maven:
[java]java.lang.Exception: Timeout scanning annotations[/java]
Unlike, with a standalone instance of Jetty, the WAR is successfully deployed.

Complete stacktrace

[java]2014-09-08 22:28:50.669:INFO:oeja.AnnotationConfiguration:main: Scanned 1 container path jars, 87 WEB-INF/lib jars, 1 WEB-INF/classes dirs in 65922ms for context o.e.j.m.p.JettyWebAppContext@13bb109{/,[file:/D:/JLALOU/development/forfait-XXX-XXX/XXX-web/src/main/webapp/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/extensions/primefaces-extensions/2.0.0/primefaces-extensions-2.0.0.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/themes/bootstrap/1.0.10/bootstrap-1.0.10.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/primefaces/5.0/primefaces-5.0.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/com/sun/faces/jsf-impl/2.2.6/jsf-impl-2.2.6.jar!/META-INF/resources/],STARTING}{file:/D:/JLALOU/development/forfait-XXX-XXX/XXX-web/src/main/webapp/}
2014-09-08 22:28:50.670:WARN:oejw.WebAppContext:main: Failed startup of context o.e.j.m.p.JettyWebAppContext@13bb109{/,[file:/D:/JLALOU/development/forfait-XXX-XXX/XXX-web/src/main/webapp/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/extensions/primefaces-extensions/2.0.0/primefaces-extensions-2.0.0.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/themes/bootstrap/1.0.10/bootstrap-1.0.10.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/org/primefaces/primefaces/5.0/primefaces-5.0.jar!/META-INF/resources/, jar:file:/C:/Users/jlalou/.m2/repository/com/sun/faces/jsf-impl/2.2.6/jsf-impl-2.2.6.jar!/META-INF/resources/],STARTING}{file:/D:/JLALOU/development/forfait-XXX-XXX/XXX-web/src/main/webapp/}
java.lang.Exception: Timeout scanning annotations
at org.eclipse.jetty.annotations.AnnotationConfiguration.scanForAnnotations(AnnotationConfiguration.java:571)
at org.eclipse.jetty.annotations.AnnotationConfiguration.configure(AnnotationConfiguration.java:441)
at org.eclipse.jetty.webapp.WebAppContext.configure(WebAppContext.java:466)
at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1342)
at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:745)
at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:492)
at org.eclipse.jetty.maven.plugin.JettyWebAppContext.doStart(JettyWebAppContext.java:282)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:117)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:99)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.doStart(ContextHandlerCollection.java:154)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:117)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:99)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:117)
at org.eclipse.jetty.server.Server.start(Server.java:358)
at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:99)
at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:60)
at org.eclipse.jetty.server.Server.doStart(Server.java:325)
at org.eclipse.jetty.maven.plugin.JettyServer.doStart(JettyServer.java:68)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.maven.plugin.AbstractJettyMojo.startJetty(AbstractJettyMojo.java:564)
at org.eclipse.jetty.maven.plugin.AbstractJettyMojo.execute(AbstractJettyMojo.java:360)
at org.eclipse.jetty.maven.plugin.JettyRunMojo.execute(JettyRunMojo.java:168)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:133)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:108)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:76)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:116)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:361)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:155)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:584)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:213)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:157)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)[/java]

Explanation

Since the release of 9.1 branch, Jetty server limits the scan time for annotations at 60 seconds, by default.
The exception is raised here: source
[java]boolean timeout = !latch.await(getMaxScanWait(context), TimeUnit.SECONDS);

if (LOG.isDebugEnabled())
{
for (ParserTask p:_parserTasks)
LOG.debug("Scanned {} in {}ms", p.getResource(), TimeUnit.MILLISECONDS.convert(p.getStatistic().getElapsed(), TimeUnit.NANOSECONDS));

LOG.debug("Scanned {} container path jars, {} WEB-INF/lib jars, {} WEB-INF/classes dirs in {}ms for context {}",
_containerPathStats.getTotal(), _webInfLibStats.getTotal(), _webInfClassesStats.getTotal(),
(TimeUnit.MILLISECONDS.convert(System.nanoTime()-start, TimeUnit.NANOSECONDS)),
context);
}

if (timeout)
me.add(new Exception("Timeout scanning annotations"));[/java]

Fix

As a quick fix, on launching Maven, add the option -Dorg.eclipse.jetty.annotations.maxWait=120 (set a higher value if needed):
[java]mvn jetty:run -Dorg.eclipse.jetty.annotations.maxWait=120[/java]

You can also set this property directly in jetty-*.xml configuration files, for webapp or even for a complete server.

Oracle XDB port runs by default on port 8080… which is quite an issue for Java web developpers, because of the collision with default port used by servlet engines such as Tomcat and Jetty.

To display Oracle XDB port, run:
[sql]select DBMS_XDB.GETHTTPPORT from dual;[/sql]

To change it (for instance to set it on port 9090), run
[sql]exec DBMS_XDB.SETHTTPPORT(9090);[/sql]

Case

On deploying a JSF 2.2 / Primefaces 5 application on Jetty 9, I got the following error:
[java]java.lang.IllegalStateException: Could not find backup for factory javax.faces.context.FacesContextFactory.[/java]

The issue seems linked to Jetty, since I could not reproduce the issue on Tomcat 8.

Quickfix

In the web.xml, add the following block:
[xml] <listener>
<listener-class>com.sun.faces.config.ConfigureListener</listener-class>
</listener>[/xml]

Case

I run a project with JSF 2 / PrimeFaces 5 (BTW: it rocks!) / Spring 4 / Jetty 9 / Java 8:
[java]MultiException java.lang.RuntimeException: Error scanning file SummerBean.class, java.lang.RuntimeException: Error scanning entry …/SummerService.class from jar file:/…/spring-tier-1.0-SNAPSHOT.jar, java.lang.RuntimeException: Error scanning entry …/SummerServiceImpl.class from jar file:/…/spring-tier-1.0-SNAPSHOT.jar
at org.eclipse.jetty.annotations.AnnotationConfiguration.scanForAnnotations(AnnotationConfiguration.java:530)[/java]

Explanation

The error occurs because of a conflict on the JARs of ASM.

Fix

You have to override Jetty’s dependencies to ASM.
In Maven’s POM, amend Jetty plugin to force ASM versions:
[xml]<plugin>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-maven-plugin</artifactId>
<version>${jetty.version}</version>
<dependencies>
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm</artifactId>
<version>5.0.2</version>
</dependency>
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm-commons</artifactId>
<version>5.0.2</version>
</dependency>
</dependencies>
<!– … –>
</plugin>
[/xml]

Then it should work 😉

Stacktrace

On deploying a WAR in Tomcat:
[java]org.apache.tomcat.util.http.fileupload.FileUploadBase$SizeLimitExceededException: the request was rejected because its size (128938160) exceeds the configured maximum (52428800)[/java]

Quick fix

Edit the file $CATALINA_HOME/webapps/manager/WEB-INF/web.xml
Replace the block
[xml] <multipart-config>
<!– 50MB max –>
<max-file-size>52428800</max-file-size>
<max-request-size>52428800</max-request-size>
<file-size-threshold>0</file-size-threshold>
</multipart-config> [/xml]

with:

[xml] <multipart-config>
<!– 200 MB max –>
<max-file-size>209715200</max-file-size>
<max-request-size>209715200</max-request-size>
<file-size-threshold>0</file-size-threshold>
</multipart-config>
[/xml]