Spring Boot @Autowired Abstract/Base Class

Properly using (and not overusing) abstract classes is vital to having a cleanly organized and DRY application.  Developers tend to get confused when trying to inject services into abstract or base classes in Spring Boot though.

Setter injection actually works fine in base classes; but as noted here, you should  make sure to use the final keyword on the setter to prevent it from being overridden.

public abstract class AbstractSchemaService {
    ...
    private StringShorteningService shortener;

    @Autowired
    public final void setStringShorteningService(StringShorteningService shortener) {
        this.shortener = shortener;
    }
    ...
}

Java – Get Parent Class Private Variable in Sub Class

I’m working on making a derived version of the Presto Hive plugin.  Unfortunately, the base class I need to inherit from uses its own private class loader, and the function I need to override (which is override-able!) for some reason requires that class loader as a parameter.

Anyway, long story short, I need to get the parent object’s private field to use it in the sub class I’m creating.  Reflection to the rescue!

Note: This is not generally good programming practice. Understand what the code does and why it does it before doing this.

Solution

//Class A file.

public class ClassA {
    private String name;
    public ClassA() {
        this.name = "Hello World!";
    }
}

// Class B file.

import java.lang.reflect.Field;

public class ClassB extends ClassA {
    public ClassB() {
        super();
    }

    public void printSuperPrivateMember() throws Exception {
        Field nameField = ClassA.class.getDeclaredField("name");
        nameField.setAccessible(true);
        System.out.println((String) nameField.get(this));
    }

    public static void main(String[] args) throws Exception {
        ClassB b = new ClassB();
        b.printSuperPrivateMember();
    }
}

Spring Time out REST HTTP Calls With RestTemplate

No Timeouts By Default!

Spring’s RestTemplate is an extremely convenient way to make REST calls to web services.  But most people don’t realize initially that these calls have no timeout by default.  This means no connection timeout and no data call timeout.  So, potentially, your app can make a call that should take 1 second and could freeze up for a very long time if the back end is behaving badly.

Setting a Timeout

There are a lot of ways of doing this, but the best one I’ve seen recently (from this stackoverflow post) is to create the RestTemplate in an @Configuration class and then inject it into your services.  That way you know the RestTemplate you are using everywhere was configured properly with your desired timeouts.

Here is a full example.

package com.company.cloudops.config;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;
import java.time.Duration;

@Configuration
public class AppConfig {

    @Value("${rest.template.timeout}") private int restTemplateTimeoutMs;

    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {
        return builder
                .setConnectTimeout(Duration.ofMillis(restTemplateTimeoutMs))
                .setReadTimeout(Duration.ofMillis(restTemplateTimeoutMs))
                .build();
    }
}

To use this RestTemplate in another Spring bean class, just pull it in with:

@Autowired private RestTemplate template;

Connection Pooling With Spring 2.0 Hikari – Verify Idle Timeouts are Working

Use Case

I’ve been working on an odd API project where each user needs their own connection to various back-end databases/data-sources.  This is a break from the norm because in general, you set up a connection pool of, say, 10 connections and everyone shares it and you’re golden.

If you have 500 users throughout the day though and each one gets some connections, that would be a disaster.  So, in my case making sure the pool is of limited size and making sure the idle timeout works is pretty vital.  So, I started playing around to see how I can verify old connections are really being removed.

My Configuration

I had started with an Apache BasicDataSource (old habits die hard).  But then when I enabled debug I didn’t see connections being dropped, or info on them being logged at all for that matter.  Before bothering with trace, I started reading about Hikari which is a connection pool I see spring using a lot… and it looked pretty awesome! See some good performance and usage info right here.

Anyway! I switched to Hikari quick which was easy since its already in Spring Boot 2.X (which I habitually use for everything these days).

Here’s my Spring config class/code. I have it set in properties to allow a minimum of 0 connections, to time out connections after 60 seconds, and to have a maximum of 4 connections. Connections are tested with “select 1” which is pretty safe on most databases.

@Configuration
public class Config {

    //Configuration for our general audit data source.
    private @Value("${audit.ds.url}") String auditDsUrl;
    private @Value("${audit.ds.user}") String auditDsUser;
    private @Value("${audit.ds.password}") String auditDsPassword;

    @Bean
    public DataSource auditDataSource() {
        HikariConfig config = new HikariConfig();
        config.setJdbcUrl(auditDsUrl);
        config.setUsername(auditDsUser);
        config.setPassword(auditDsPassword);
        config.setMaximumPoolSize(4);
        config.setMinimumIdle(0);
        config.setIdleTimeout(60000);
        config.setConnectionTestQuery("select 1");
        config.setPoolName("Audit Pool");
        config.setValidationTimeout(10000);
        return new HikariDataSource(config);
    }

    @Bean
    public NamedParameterJdbcTemplate auditJdbcTemplate() {
        return new NamedParameterJdbcTemplate(auditDataSource());
    }
}

Verifying it Works

After sending a query to my API, where it uses a basic JDBC template to execute the query, I see the logs do this (note that I removed the date/time/class/etc for brevity).

Audit Pool - Before cleanup stats (total=0, active=0, idle=0, waiting=0)
Audit Pool - After cleanup stats (total=0, active=0, idle=0, waiting=0)
Audit Pool - Before cleanup stats (total=1, active=0, idle=1, waiting=0)
Audit Pool - After cleanup stats (total=1, active=0, idle=1, waiting=0)
Audit Pool - Before cleanup stats (total=1, active=0, idle=1, waiting=0)
Audit Pool - After cleanup stats (total=1, active=0, idle=1, waiting=0)
Audit Pool - After cleanup stats (total=0, active=0, idle=0, waiting=0)
Audit Pool - Closing connection PG...: (connection has passed idleTimeout)

So, we can see that it went from 0 connections total, to 1 connection total. The connection looks idle pretty quick because it was a short query that was done before the regular output log. Then after a minute, the connection gets closed and the total goes back to 0.

So, we’re correctly timing out idle connections using our settings. Also, we’re getting our pool name (Audit Pool) in the logs which is awesome too!

Java Algorithm: Pascal’s Triangle

Pascal’s Triangle

Pascal’s triangle is a problem where you want to print a triangle of a certain height where each element is the sum of the 2 elements above it.  The first row is 1, the second row is 2 1’s, and then the pattern builds from there with 1 on the ends and the other elements being the sum of their parents.

For Example:

        1
       1 1
      1 2 1
     1 3 3 1
    1 4 6 4 1

Generalized Solution

It’s always good to (first) try to solve algorithms yourself without looking at other peoples’ solutions so that you truly learn how to work them out yourself in real scenarios.

So, there may be a more efficient solution than this; but here was my approach:

  • Set a list to hold the previous row (initially empty).
  • Loop up to the required depth from 1 to D inclusively.
    • Loop for each item that should be in that level (level 1 has 1 number, level N has N numbers).
    • If it’s an end-number add “1” to the new row, otherwise add the sum of parents.
  • Print the new row.
  • Store the new row as the previous row so it can be used for the next depth level’s parent calculations.

I’m sure you can do this without storing the previous row as well mathematically, but this is pretty elegant and will only take extra space equal to the sizeof(int) * level-number which is really nothing.

Java Solution

package john.humphreys;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class PascalsTriangle {

    public static void main(String[] args) {
        printToDepth(20);
    }

    private static void printToDepth(int d) {

        //Row 1 has value 1, anything less is invalid.
        if (d < 1) return;

        //Keep track of the previous row.
        List<Integer> previousRow = new ArrayList<>();

        //Loop from 1 to target depth inclusively.
        for (int i = 1; i <= d; ++i) {

            //Create a new row to populate with our solution.
            List<Integer> newRow = new ArrayList<>();

            //If this is a row-end (0 or max in row) add 0, otherwise add the parents' sum.
            for (int ri = 0; ri < i; ++ri) {
                newRow.add(ri == 0 || ri == i - 1 ? 1 : previousRow.get(ri - 1) + previousRow.get(ri));
            }

            //Print out the space-separated row.
            System.out.println(newRow.stream().map(Object::toString).collect(Collectors.joining(" ")));

            //Store this as the previous row.
            previousRow = newRow;
        }
    }
}

If we take out comments, gratuitous spacing, and imports, it’s quite lean:

public class PascalsTriangle {

    public static void main(String[] args) {
        printToDepth(20);
    }

    private static void printToDepth(int d) {
        if (d < 1) return;
        List<Integer> previousRow = new ArrayList<>();

        for (int i = 1; i <= d; ++i) {
            List<Integer> newRow = new ArrayList<>();
            for (int ri = 0; ri < i; ++ri) {
                newRow.add(ri == 0 || ri == i - 1 ? 1 : previousRow.get(ri - 1) + previousRow.get(ri));
            }
            System.out.println(newRow.stream().map(Object::toString).collect(Collectors.joining(" ")));
            previousRow = newRow;
        }
    }
}

Java Regex Capture/Extract Multiple Values

Use Case

When you’re trying to parse complex log lines or extract data from complex strings, regular expression capture groups are about the most useful tool you could possibly ask for.

This example is taken from work where I had to parse and analyze some logs for loading data to a database. A log sample would look like this:

/data/SXF_SX_4906_2019-04-13.01.43.24.143.log:2019-04-13 01:43:28,320 INFO com.x.dc.db.schemagen.batch.listener.JobResultListener [tx.id=IF-TX-ID-a23c195c-673a-47ab-ab0c-7b8591821169] [main] Inside sendEmailNotification method: subject is prod alert:DB copy job STARTED for the dataset:4906

The Code

The relevant part of the code is here:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

private static final String capturePattern =
"^/.*/SXF_SX_(\\d+)_(\\d{4}-\\d{2}-\\d{2}.\\d{2}.\\d{2}.\\d{2}.\\d{3}).log:(.*) INFO.*" +
"copy job (.*) for the dataset:.*"

//Leaving out rest of class, this is just the regex parsing portion.
//isValid, fulLLogEntry, dataSetId, fileTimestamp, logTimestamp, status are all
//member variables in a class where this function is a member.
public DbLoadLog(String line) {

    isValid = true;

    Pattern r = Pattern.compile(capturePattern);
    Matcher m = r.matcher(line);

    //If you wanted to run over a multi-line-string/file, you could put
    //m.find() in a while loop and keep going; but I'm just analyzing specific lines.
    if (m.find()) {
        fullLogEntry = line;
        dataSetId = Integer.valueOf(m.group(1));
        fileTimestamp = m.group(2);
        logTimestamp = m.group(3);
        status = m.group(4);
    }
    else {
        isValid = false;
    }
}

 

Java – Regular/Scheduled Task, One Run at a Time

This will be a very short post, and I’m mostly writing it just so it sticks in my head better.

Common Use Cases

There are many times when you may find that you need to regularly run a task in Java.  Here are a few common examples:

  • You have a cache you need to refresh every X minutes to power a dashboard or something similar.
  • You need to prune old files from a file system once an hour.
  • You need to regularly update stats counters for monitoring.

Coding Options

There are a lot of ways to do this, but the recommended approach would be to use a scheduled executor.  Now… this part is easy to remember, but what is sometimes hard to remember is that you have two options when scheduling a task.  I often find myself picking the wrong one as it pops up in Intelli-sense and I forget there are 2 options.

  1. Run the task every X seconds/minutes/etc no matter what.
  2. Run the task every X seconds/minutes/etc *after* the previous task completed.

These two things can be very different.  If you have a task that only takes a couple of seconds, it probably doesn’t matter much.  But if you have a task that takes 2 minutes and you’re running it every 1 minute, then with option 1 you will always be running at least 2 copies of the task, whereas with option 2 you’ll just be running one copy at a time with a minute of buffer in between each task.

For both options, you can create the scheduled executor service the same way:

ScheduledExecutorService se = Executors.newSingleThreadScheduledExecutor();

But for option #1 (run every interval regardless of previous tasks), you would use this function:

se.scheduleAtFixedRate(this::refreshCache, 10, 120, TimeUnit.SECONDS);

And for option #2 (start counting after previous task completes), you would use this function.

se.scheduleWithFixedDelay(this::refreshCache, 10, 120, TimeUnit.SECONDS);