Avoid Publishing SNAPSHOTs to Nexus

3. July, 2018

Once in a while I’m running into people who deploy SNAPSHOT versions of Maven dependencies to a company-wide Nexus server with a job on a CI server. This is usually a very bad idea, especially when using branches.

Scenario: Two developers, John and Mary, each working in their own branch. They push their branches, CI builds them and they end up on Nexus.

Problem: Nexus doesn’t know or care about branches. Whichever job finishes last wins.

Often, this is not a problem. Now let’s add another project B. B depends on A.

As long as B depends on a release of A, everything is fine.

Now, John needs to make some changes in A. So he updates the dependency in B to A-x.y.z-SNAPSHOT. Everything is still fine, since Mary still uses the latest release of A.

Then Mary also creates a feature branch in her clone of A. That still doesn’t break anything because Maven caches SNAPSHOTs for a day.

The next morning, John makes a change to B and builds it.

This build might break when Mary’s CI job finished last!

The problem here, which can go unnoticed for years, is that Maven silently downloaded Mary’s version of A onto John’s computer and used that to compile. John will see the source code from his branch of A but the binaries will be something else.

Eventually, one of them will make a small changes which affects the others project. They will see MethodNotFoundException or get strange compile errors while the source code (which isn’t affected by this) will look perfectly fine or unit tests will break in odd ways.

That is the main reason why you shouldn’t deploy SNAPSHOT branches to a shared Maven repository: It creates a small chance for subtle bugs which will take a long time to find since your mental model (“I see the source, this is what I get”) will be wrong.

You can get away with publishing the master branch to Nexus (i.e. only a single branch with SNAPSHOTs will ever be published to Nexus).

Note: If your CI server shares local Maven repositories between projects, your builds can fail on the CI server for the same reason. Configure your CI server for per-project local repositories and wipe them before the build to avoid such issues for sure.

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Posted by digulla

Binding one Instance to Two Interfaces in Guice

15. June, 2018

Sometimes, you need to bind one instance to two interfaces in Guice, for example:

interface Foo { ... }

interface Bar extends Foo { ... }

class FooBarImpl implements Bar { ... }

Let’s imaging FooBarImpl is a stateful bean which is saved in a session scope:

bind(Bar.class)
    .to(FooBarImpl.class)
    .in(Session.class);

This works when someone injects Bar but injecting Foo fails. There are two solutions for this: Bind Foo to Bar or use a provider method to transform the type.

Solution 1: Binding Interfaces

bind(Foo.class)
    .to(Bar.class);

This will use the mapping for Bar to satisfy requests for Foo.

Solution 2: Provider Method

Provider methods are a nice way to build complex objects from bound instances. In our case, we can map the types like so:

@Provides
@Session
public Foo fooProvider(Bar impl) {
    return impl;
}

Note: These approaches will put two keys into your scope but they will point to the same instance.

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Posted by digulla

Stop on specific NullPointerException in Eclipse

20. December, 2017

TL;DR: Find the line where the exception occurs and then add a conditional breakpoint which checks the local variables for values that would trigger the exception.

In Eclipse, you can stop on any exception by using the menu Run -> “Add Java Exception Breakpoint…“.

That’s great but it doesn’t help when you want to stop on a certain exception on a certain line. It’s a big problem with code that uses exceptions for control flow – you could have thousands of those exceptions before you get to the one which you care about.

In my case, I had a NullPointerException in java.util.concurrent.ConcurrentHashMap:

    public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode()); <-------- key is null
        ...

I tried to add a conditional breakpoint here with the condition key == null but Eclipse complained that it couldn’t compile the expression (probably missing debug information).

The method was called from Jetty’s ClassInheritanceHandler, so I added a conditional breakpoint there.

That’s another reason to copy method results into local variables before using them.

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Posted by digulla

YouDebug, the Non-interactive Debugger

7. May, 2014

“Non-interactive” and “debugger” seem to be contradictory but bear with me. YouDebug is a tool which allows you to debug a running Java application with a script.

How could this be useful? From the website:

[…]your program fails at a customer’s site with an exception, but you can’t (or don’t want to) reproduce the problem on your computer[…]

Sounds like a familiar problem: Customer has a problem but they can’t give you access for security, legal or technical reasons. You can’t go there (too far away) of even if you could, security won’t let you touch anything.

In a nutshell, YouDebug is a debugger that is controlled by a Groovy script:

breakpoint("com.acme.SubStringTest",7) {
  println "s="+s;
}

This sets a breakpoint in line 7 of “SubStringTest” and then prints the value of the local variable “s”.

Granted, it’s more time-consuming then doing it yourself (and you may need several attempts to get at the bottom of things) but you don’t have to install an IDE at your customer site, you don’t have to bring the source code along and technically, the customer is already running code that you wrote so from a legal and security point of view, this isn’t much different.

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Posted by digulla

Oracle’s Java FAQ Displays in Chinese

17. January, 2014

I just looked at an article in the Oracle Java FAQ and I was a bit surprised to get this result:

         如何設定例外網站清單？

本文適用於:

  • Java 版本: 7.0

從 Java 7 Update 51 版本開始導入了例外網站清單功能。藉由將應用程式 URL 新增至「例外」清單，可讓使用者執行通常會被安全檢查封鎖的「豐富網際網路應用程式 (Rich Internet Applications, RIA)」。[…]

The solution was pretty simple: Append ?locale=en to the end of the URL: http://www.java.com/de/download/faq/exception_sitelist.xml?locale=en

That fixed it for me … sometimes.

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Posted by digulla

Bean of type is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)

30. October, 2013

Haunted by this? Getting mysterious NullPointerExceptions in your BeanPostProcessors?

I have written a lengthy answer how to debug and solve these issues on stackoverflow.com.

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Posted by digulla

Jazoon 2013 – The Economies of Scaling Software

25. October, 2013

Jazoon 2013 badgeIf you’re small, scalability is not an issue. If you’re big, you can plan for it. But what if you wake up one morning and you suddenly find you’ve become the next Google?

In his talk “The Economies of Scaling Software“, Abdelmonaim Remani talks about what scaling means, how you can plan for it (even if you don’t expect it to happen) and all the nasty details that stand between you and success (slides on slideshare).

Today, the ubiquity of the Internet has blurred the lines between consumers and enterprises. They start to ask the same questions which eventually all boil down to: “How can I find what I’m looking for in a universe of haystacks?”

One day, many of us will find themselves with the need to scale because the performance of the old solution has become unbearable and all low-hanging fruit (faster CPU, more RAM) have been picked.

To solve this, you can look at the CPU (slide 16-35) or you can start to build clusters (36-45).

Or your I/O might be the bottleneck (slides 46-60). You can solve this by looking at NoSQL databases and caching.

Is the network the problem? (slides 61-75) Start with asynchronous processing, batch jobs, content delivery networks (CDN), DNS sharding or use a different protocol to connect the various parts of your system.

But how do you know what part is the bottleneck? The answer here is monitoring. (slide 77)

Note that scaling often helps with disaster recovery, you still have to plan for it – if all nodes of your cluster are in the same room, it’s still a single point of failure (slide 79)

Software isn’t everything. Don’t forget your team (slide 82)

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Posted by digulla

Jazoon 2013 – Join the Java Evolution; JCP & Adopt-a-JSR

25. October, 2013

Jazoon 2013 badgeHeather VanCura explained how the new JCP works in her talk “Join the Java Evolution; JCP & Adopt-a-JSR” (slides on slideshare)

Oracle spent the last years to make the JCP much more open and accessible. One example here are the JSRs which are much more lightweight than the complex JCPs. You can even adopt one.

Maybe you’re a long time joda-time user and want to make sure important features make it into JSR 310 – new Date and Time API for Java? Join them to discuss your need, share some code, help write, improve or translate documentation.

Some bug in the SDK nagging you? Contribute a fix to the OpenJDK. With Java 8, the OpenJDK build has been simplified tremendously. Sun ignored your bug report for years even though it contained a patch? Now is the time to change this.

I had the chance to chat with Heather after the talk which earned me a copy of “Java 7 Concurrency Cookbook”; thanks for that 🙂 We discusses a couple of ideas and she gave me points; maybe I’ll submit a few patches. If I do, I’ll blog about the experience here.

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Posted by digulla

Jazoon 2013 – Kafka and Storm – event processing in realtime

25. October, 2013

Jazoon 2013 badgeGuido Schmutz introduces two frameworks in his talk “Kafka and Storm – event processing in realtime” (slides on slideshare)

Apache Kafka is a publish-subscribe messaging system like JMS but more simple. Messages are kept in files and never deleted. Together with the fact that subscribers have to tell the system which message they want next, this means you can recover from bugs that corrupted your data even if you notice them only after some time: Just process all the corrupted messages again.

Storm is a “distributed realtime computation system. ” It makes it easy to define topologies (= graphs) of bolts (= places where a computation takes place) flowing your real-time data through a complex network to process, filter and aggregate it. Just like Akka, it defines all kinds of operations (filters, switches, routers, …) so you can easily and quickly build the topology you need. Trident makes this set-up step even more simple.

Compared to Hadoop, Storm is meant for real-time processing. Some projects combine the two.

If you need a good framework for serializing data in Java, have a look at Apache Avro.

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Posted by digulla

Jazoon 2013 – Rocket Propelled Java

25. October, 2013

Jazoon 2013 badge“Rocket Propelled Java” is about being more productive by writing less boilerplate code (slides on slideshare).

In his talk, Andres Almiray showed some tools to make your code more compact while keeping or even improving readability.

The first one is lombok and lombok-pg which use Java annotations to generate a lot of boiler plate code (slides 3-6)

Similar, you can use Groovy AST transformations to create things like immutables or singletons correctly (“There are 5-6 ways to create singletons. Only two of them are correct”, slides 7-9)

Note: You might be worried to add Groovy to a pure Java project. You don’t have to. Just use it for the AST transformations to stop wasting time on writing code that only makes your compiler happy. Use Groovy as a “boilerplate code buster.”

If you ever wanted to use “Design by Contract“, GContracts is for you.

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Posted by digulla

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