[Android] Sprescia app for run activity tracking

No it is not Christmas today, but here is another free, no ads, no trackers, no data collection, minimalistic app to track your run activity!

Built for Android 14+ using help from ChatGPT and icons from SVG Repo this app writes to a local Rooom DB in 1 table: run

Run view allows you to add, edit, delete run details and calculates average speed for each entry, comparing it with the previous one, giving you a quick view of your training progression. You can also export and import data to/from csv for quick backup and restore logic.

Daily stats view allows you to see daily run data and compare it to previous runs using key metrics: steps, average speed, distance, time.

Monthly stats view provides the same capability but averages the data grouping per month instead.

You can find the Sprescia project on my GitHub

[Android] MaiCar app for car expense management (refuels and maintenance)

Finally a free, no ads, no trackers, no data collection, minimalistic app to track your car expenses!

Built for Android 14+ using help from ChatGPT and icons from SVG Repo this app writes to a local Rooom DB in 2 tables: fuel and maintenance

Fuel view allows you to add, edit, delete refuel details (partial and full tank) and calculates efficiency for each entry, comparing it with the previous one, giving you a quick view of your trip efficiency progression.

Maintenance view allows you to add, edit, delete maintenance details for multiple maintenance types, it also allows you to filter the view on a specific maintenance type to quickly find a specific item.

Both views allow you to export and import data to/from csv for quick backup and restore logic.

You can find the MaiCar project on my GitHub

[Java] Get type of elements in collection using reflection

Java type erasure logic means that at runtime some information is removed/replaced from generic declarations, which poses a minor challenge when trying to identify the actual parameter types using reflection at runtime.

Here is a way:

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;


//maybe you are looping over fields or have a field of type Field f
ParameterizedType collectionType = (ParameterizedType) f.getGenericType();
Class<?> actualClassOfElementInCollection = (Class<?>) collectionType.getActualTypeArguments()[0];

[Java] Load entity with lazy children collections

When using JPA and lazy collection loading, at the time the parent entity is retrieved from DB, the lazy children are NOT also loaded in memory, instead, a proxy reference is added, which is used to retrieve the data if that particular field is ever accessed.

In some scenarios you might want to load the whole entity, including the lazy children in memory instead (eg you want to clone/serialize it, whatever).

If you want to load ONE child together with the parent, a JOIN FETCH clause would do the trick: 

SELECT p

FROM Parent p

LEFT JOIN FETCH p.childField c

WHERE p.id = :id

But if you try to load more than one child at the same time, you will get a MultipleBagFetchException. A workaround is to call the load with JOIN FETCH for all entities you need sequentially, for example:

private Parent loadChildViaQuery(ID id, String childClause) {
  return entityManager
    .createQuery(
      "select p " +
      "from Parent p " +
      "left join fetch " +
      childClause +
      " where p.id = :id",
      Parent.class
    )
    .setParameter("id", id)
    .getSingleResult();
}

Where childClause input is the join statement you need, for example:

"p.childField c"

Also remember that if the parent entity is not found, the operation would throw a NoResultException, while if the child is not found, no exception is raised, the child is simply null/empty.

[Java] Using annotation processing (and validating it) to execute logic at runtime

Sample project showcasing how to use annotations to perform runtime logic to log changes in object values.

Remember it is a SAMPLE, so obviously (lazy me) most null-safe checks and so are not included and obviously some logic is a showcase, should be replaced with a real business scenario to implement.

In this SAMPLE, we tag fields to be included in a diff logic to then print to output when those fields values change by comparing two instances of the same object.

Logic can obviously be made much more complex including collections and maps and whatnot (Java Generics are your friends there).

Also worth mentioning JaVers can do most of it for you, unless you have fancy business requirements that force you to write custom code..

Key points:

- How to create an annotation

- How to create an annotation processor to validate the annotation parameters at compile time

- How to register an annotation processor

- How to configure a multi-module Maven project to use a custom annotation processor (also, in the pom of the root project ensure the module containing the processor is built BEFORE everything else)

- How to test an annotation processor by generating classes at runtime and trigger compilation tasks agains them. Includes verifying compilation warnings are properly triggered as expected.

- How to use reflection to get fields annotated with a given annotation (and then execute whatever logic on them)

- How to use reflection to invoke methods (including static methods)

The full project is available on my GitHub repo with commented code: https://github.com/steghio/diff-annotation-processing

[Java] Serialize POJO to XML according to XSD

If you generate a class from an XSD schema, it will come with the necessary annotations to serialize it to an XML String.

You can therefore easily convert it with:

import jakarta.xml.bind.JAXBContext;
import jakarta.xml.bind.Marshaller;
import java.io.StringWriter;

/**
 * Provides utility methods for serialization scenarios
 */
public class SerializationUtils {

  /**
   * Serialize the given object to XML String using JAXBContext
   * It will set the output to be pretty printed
   * It relies on the object annotations to correctly place and annotate all fields
   * @param object
   * @return the string representation of this object as XML
   * @param <T>
   */
  public static <T> String serializeXml(T object) {
    try {
      JAXBContext jc = JAXBContext.newInstance(object.getClass());

      Marshaller marshaller = jc.createMarshaller();
      marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
      //to completely remove the xml preamble `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>` add this line:
      //marshaller.setProperty(Marshaller.JAXB_FRAGMENT, true);

      //marshaller cannot output to string directly
      StringWriter sw = new StringWriter();

      marshaller.marshal(object, sw);

      return sw.toString();
    } catch (Exception e) {
      throw new RuntimeException("Failed to convert payload to xml. ", e);
    }
  }
}

[Java] Generate POJO from XSD in Maven

Assuming you have a nice correct XSD file with proper namespace references and all, then you could convert it to a POJO (or more) using jaxb-maven-plugin

There are multiple plugins that would achieve the same result and multiple versions of this plugin even, so searching on the web can be confusing. In year 2024, this works simply with adding a plugin in the POM:

<plugin>
  <groupId>org.jvnet.jaxb</groupId>
  <artifactId>jaxb-maven-plugin</artifactId>
  <version>4.0.8</version>
  <executions>
    <execution>
      <id>NAME_FOR_THIS_RUN</id>
      <goals>
        <goal>generate</goal>
      </goals>
      <configuration>
        <schemaDirectory>src/main/resources/FOLDER/USE_CASE</schemaDirectory> <!-- here will be the XSD -->
      </configuration>
    </execution>
  </executions>
</plugin>

[Java] Prim algorithm to find Minimum Spanning Tree in a graph

The minimum spanning tree (MST) is a subset of all edges in a weighted, undirected, connected graph such that the resulting graph is still connected and the sum of all edge weights is minimal.

If we are not given a list of edges, but only a list of vertices and a formula to calculate the edge weight given two graph nodes, we can run a preprocessing step to generate ALL possible edges between ALL graph nodes and calculate their weight in O(V^2).

Then, starting from a random node, we greedily choose one reachable vertex which has minimal distance from the current node. We continue exploring until all graph nodes have been touched.

There might be multiple valid MSTs for a given graph, this algorithm will return one of them.

We use a queue sorted by weight to determine which edge (and therefore node) to visit next, this ensures that if a node is reachable via multiple edges, we always pick the smallest weight for it. Since the graph is fully connected, we are ensured eventually we will have picked ONE edge between each node in the graph, and the sum of weight of all the chosen edges is minimal.

This runs using O(E) space since we might add all edges to the queue and O(E log(E)) time since for each edge we add to the queue we pay the O(log(E)) cost of insert and remove operation.

If the graph was NOT connected, this will NOT return the MST, only the MST for the connected component where the chosen start node resides. We could adapt the algorithm to verify whether there are extra nodes not yet visited, and repeat the processing for each until we have created a MST for each connected component in the graph.

In case the graph is not conencted, an alternatve can also be Kruskal's algorithm.

You can find my implementation of primMinimumSpanningTree on my Gist along with some tests in PrimMSTJTests.

[Java] Graph union find algorithm

For an undirected graph, we can compute the disjointed sets that represent all connected nodes in the subgraph where each node resides.

For each set, we elect a representative, all nodes reachable in a set will have the same representative. The resulting view will be a tree where the representative sits at the root and all connected nodes are its children.

Example applications include: quickly verify whether 2 nodes in a graph have a path to each other (they must belong to same set) or calculating the minimum number of edges to add to a graph to make it fully connected (or the opposite).

It is based on 2 operations:

find(Vertex x)

which will return for a given node, the representative of its subset. We recurse up the tree where this node resides until the representative is found. We optimize the operation for future searches by including path compression, where once a representative is found, all nodes along the same path are updated to track it. This makes it so that the find operation runs in O(inverse Ackermann(V)), which is considered O(1) but more realistically is O(log(log(...(V))) or how many times we need to apply log(x) to its result starting with V until the output is less than 1. It is an extremely slowly increasing sequence.

union(Vertex x, Vertex y)

which will connect the subtree where node x resides to the subtree of node y, unless they are already part of the same subtree. To improve efficiency we track in O(V) extra space the rank of each subtree (its depth) and when merging two subtrees, we connect the one with minimum depth to the other, so the overall height of the resulting tree is kept as flat as possible.

It uses O(V) extra space, to track for each node who is the representative of its subset and O(V) to track the rank of the subtree rooted at each node.

It runs in O(V Ackermann(V)) time since we run 2 find operation for each edge (pair of nodes) we unite and use the union by rank with path compression method.

You can check my implementation of unionFind on my Gist along with some tests in UnionFindJTests.

[Java] Workflow management in microservices

Also published on Medium.

[Spring] Synchronized methods and separate transactions

When marking a method as synchronized, it's important to remember that synchronization does NOT change the transactionality settings of the current operation.

For example the following parallel execution:

methodA() calls syncMethod() then calls somethingElse()
methodB() calls syncMethod() then calls somethingElse()

What happens:

1. methodA opens Transaction A enters syncMethod
2. methodB opens Transaction B waits on syncMethod
3. methodA exits syncMethod, starts executing somethingElse
4. methodB enters syncMethod while methodA completes somethingElse and commits Transaction A
5. methodB exits syncMethod, starts executing somethingElse
6. methodB completes somethingElse and commits Transaction B

Now the important part is points 4. If the syncMethod requires fresh data from other executions, methodB will NOT see that data when it enters syncMethod since Transaction A has NOT yet been commited.

The synchronization is only blocking a thread from entering the protected code block, but as soon as the resource is free, the next waiting thread will enter immediately.

This means that to allow other methods to read latest data while in the protected section, any operation done in the synchronized method must be executed in a separate transaction, for example using the TransactionHandler helper.

[Spring] Execute method in separate transaction

In Spring annotating a method as requiring a new transaction will not work if the caller of the method is in the same class.

We can easily work around this issue by creating a new class that will execute a given method in a new transaction:

 import org.springframework.stereotype.Service;  
 import org.springframework.transaction.annotation.Propagation;  
 import org.springframework.transaction.annotation.Transactional;  
   
 import java.util.function.Supplier;  
   
 /**  
  * Since spring ignores transaction settings for methods within the same class, we need a separate service  
  * to run isolated transactions which can be called from anywhere simply by supplying the method to execute  
  */  
 @Service  
 public class TransactionHandlerService {  
   
   /**  
    * Runs the given method in a the same transaction as the caller  
    *  
    * @param supplier the method to execute  
    * @param <T>  
    * @return the result of the invoked method  
    */  
   @Transactional(propagation = Propagation.REQUIRED)  
   public <T> T runInSameTransaction(Supplier<T> supplier) {  
     return supplier.get();  
   }  
   
   /**  
    * Runs the given method in a separate transaction  
    *  
    * @param supplier the method to execute  
    * @param <T>  
    * @return the result of the invoked method  
    */  
   @Transactional(propagation = Propagation.REQUIRES_NEW)  
   public <T> T runInNewTransaction(Supplier<T> supplier) {  
     return supplier.get();  
   }  
 }  

Then ensure the callers are annotated as @Transactional and simply pass the method to execute as input to our, for example:

transactionHandlerService.runInNewTransaction(() -> myMethod(someInput));

[Docker] Redirect app log file to container output

When running applications in containers, we might have log files that are relevant to monitor but are not automatically printed with the container output.

A workaround for this is to force the redirect in our Dockerfile by adding a symbolic link:

RUN ln -sf /dev/stdout /path/to/logfile.log 

Can also be applied to /dev/stderr of course

[Docker] Set Java UTF-8 encoding

Sometimes a Java based app running in a container requires UTF8 encoding. Not all available images enable that by default, this can be fixed by adding one line to your Dockerfile setting the JAVA_TOOL_OPTIONS environment variable:

ENV JAVA_TOOL_OPTIONS -Dfile.encoding=UTF8

This also works outside containers of course.

[Docker] Multi stage builds

I've been recently introduced to a nice Docker feature: multi-stage builds.

The idea is simple, if the build is containerized as well, the build itself is a developer responsability as well and the operations team need only provide a build server with docker installed on all worker nodes.

To achieve the result, we use a simple Dockerfile where we specify multiple FROM statements and tag each layer as necessary. The last layer will be the one responsible to run the application, while the previous layers are only used for the build. A sample file for a SpringBoot app looks like this:

 # syntax=docker/dockerfile:1  
 # build layer  
 FROM adoptopenjdk/openjdk11:latest as build  
   
 WORKDIR /app  
   
 # copy project files into container workdir  
 COPY . .  
   
 # build jar, skip tests, avoid daemon  
 RUN ./gradlew build -x test --no-daemon  
   
 # run layer  
 FROM adoptopenjdk/openjdk11:latest as prod  
   
 WORKDIR /app  
   
 #copy fat jar from previous layer into current workdir and rename it  
 COPY --from=build /app/build/libs/*.jar ./myApp.jar  
   
 # not mandatory, must use -p 8080:8080 later anyway  
 EXPOSE 8080  
   
 # start the spring boot app  
 CMD ["java", "-jar", "myApp.jar"]  
   

Then it can be placed in the project directory and we can trigger the build with:

docker build -t TAG .

Finally run it with (binding for example port 8080 and executing it in background):

docker run -p 8080:8080 -d TAG

We can also see the container output with (find container name with docker ps first):

docker logs -f CONTAINER_NAME

[Java] Kerberos login and retrieve GSSCredentials from KerberosTicket

A scenario that popped up recently was to login a user via Java code to Kerberos and retrieve a GSSCredential object containing the Kerberos ticket. I used Java 8, but this works since Java 7 onwards.

Java offers a Krb5LoginModule class which can be used in conjuction with a LoginContext to achieve this.

The flow is quite simple (once you have read all the Kerberos documentation):

• on the machine where the code runs, place a correct krb5.conf file in the default location for your OS (Windows uses krb5.ini) OR set the java.security.krb5.conf system property pointing to the file
• define a PasswordCallback handler class
  
• create a LoginContext with a configuration using Krb5LoginModule and provide the password callback handler. The configuration must force the login to request a user input, which will then be routed to the callback handler. It is possible to use a keytab or cache credentials, but it's not shown here
  
• login the user and get its KerberosTicket
• create a GSSCredentials object using the ticket

This procedure allows handling multiple login mechanisms in the application and even multiple Kerberos realms.

[Java] Calculate the angle between clock hands

return null;

Unless it's an analog clock, in which case:

The hour hand makes a 360 degree turn every 12 hours or 12*60 = 720 minutes, therefore each minute the hand moves 360/720 = 0.5 degrees

The minute hand makes a 360 degree turn every hour or 60 minutes, therefore each minute the hand moves 360/60 = 6 degrees

Setting 12 as the 0 position, to calculate the angle of each hand we can:

•     hours = 0.5 * current minutes for that hour (60 * hours + minutes)
•     minutes = 6 * minutes

We now have the position of both hand with respect to the 12 o'clock, the angle between them will simply be the difference, between the two positions (absolute value!)

If the difference is more than 180 degrees, the angle is a reflex angle and since we want the smallest angle between the hands, we take its complement by subtracting it from 360 (again, absolute value unless we did a modulo before to account for 24 hour format)

You can check my implementation of clockAngle on my Gist along with some tests in ClockAngleJTests.

[Java] Distribute items in K buckets

Something simple which comes up often when problems require some sort of item distribution.

Given an amount of items N and K buckets, distribute the items in the buckets as uniformly as possible.

Amount of items each bucket would hold in perfect distribution: N/K (integer division)

Amount of remaining items which could make some buckets hold extra items compared to others: N%K

Therefore simply place N/K items in each bucket, then calculate the remainder N%K and place 1 item in each bucket until remainder is 0.

Of course if K is bigger than N, some buckets will be empty.

[Java] Number of trailing zeroes in factorial

Given a number N, return the amount of trailing zeroes in its factorial.

Example:

4! = 24, no trailing zeroes

10! = 3628800, two trailing zeroes

100! = something big with 24 trailing zeroes

We could compute the factorial then count the zeroes, which is O(N) time and might work for small values of N, however multiplication can be indeed expensive.

[Java] Generate the K-th number made only of primes 3, 5, 7

Generate the K-th number made only of primes 3, 5, 7.

Expected results up to K = 15: 1, 3, 5, 7, 9, 15, 21, 25, 27, 35, 45, 49, 63, 75, 81

This exercise is one of those that once the solution is known it's extremely easy but getting to the optimal solution requires work.

An initial suboptimal idea could be to start with a queue and placing only 1 in it.

Then execute some iterations where the top of the queue is polled, multiplied by 3,5,7 and then all results are added back to the queue, including the polled value.

We need to track generated values to avoid duplicating them. After some iterations, sort the queue, then poll until the k-th value is reached.

However the numbers generated this way quickly stray from the natural ordering so we end up generating more numbers that we need to get the actual solution.

After a long examination and paper examples (try drawing a matrix that expands with each new generated value and keep generating until the solution is reached), we notice that the emerging pattern is:

• use a queue for 3s, 5s, 7s 
• take the smallest top from all queues
• multiply this value by 3,5,7 UNLESS it is bigger that the top of a queue, in which case we skip that queue, then push it back to the corresponding 3,5 or 7 queue
• stop at k-th iteration, the min value generated at that step is the answer

This uses O(N) time and space and most importantly stops as soon as a result is reached.

You can check my implementation of generateKth357 on my Gist along with some tests in GenerateKth357JTests.