Java on traviscj/blog

Transacter and Intent/Result

Wed, 15 Jun 2022 08:07:47 -0500

At SQ, we had a family of Transacter interfaces:

public interface Transacter {
  void transaction(Function<Session, Void> session);
  
  public interface Session {
    DSLContext dsl();
  }
}

where DSLContext is a jOOQ concept & is the handle for doing work against the database. Then this would be used in some class like

public class KvDao {
  @Inject Transacter transacter;
  
  void put(String ns, String k, String v) {
    transacter.transaction(session -> {
        KvRecord record = new KvRecord(ns, k, v);
        session.dsl().insertInto(KV).set(record).execute();
    });
  }
}

The parameters ns, k, and v are shorthand for namespace, key, and value, respectively. We’ll show how these might be used together shortly!

interceptors

Mon, 18 May 2020 15:13:00 -0700

INTERCEPTORS ARE SO COOL!

Sometimes you need some “generic rails” that are still highly adaptable to other uses. This is the basic problem solved by the Interceptor pattern.

I really love the way OkHttp does interceptors for the generic rails of making HTTP calls, so I wanted to walk through a case study of why an interceptor might be useful and then try to synthesize some lessons & a minimal example of the pattern.

pattern: extract mutating variables to State class

Mon, 07 Jan 2019 00:00:00 +0000

I’ve come to really like the pattern of extracting local or instance variables into their own State class, and writing an algorithm in terms of that State.

A really trivial (if contrived) example of this would be a “sum cubes” algorithm; something like this:

public class CubeSummer {
  private final PrintStream printStream;
  private long i;
  private long sum;
  
  public CubeSummer() {
    this.printStream = System.out;
    reset();
  }
  
  public long sumCubes(long limit) {
    for (; i<limit; i++) {
      sum += i * i * i;
      display();
    }
    return sum;
  }
  
  public void reset() {
    i = 0;
    sum = 0;
  }
  
  public void display() {
    printStream.print("i = " + i + ";");
    printStream.println(" sum = " + sum);
  }
}

What’s happened here is we’ve essentially promoted some variables that should arguably be local variables into instance variables, to avoid needing to pass them around explicitly. But in the process, we’ve also mixed long-lived state with short-lived state, broken thread safety, and forced clients of this class into a particular calling convention (either instantiate an instance per sumCubes call or carefully respect thread safety and call #reset().)

robustness principle and mocks

Wed, 11 Jul 2018 11:53:54 +0000

The Robustness Principle (or Postel’s Law) states

Be conservative in what you do, be liberal in what you accept from others (often reworded as “Be conservative in what you send, be liberal in what you accept”).

This principle has some criticisms.

I realized this has interesting implications for mocks. Suppose you have

public class MyObj {
  private final Integer x;
  
  public MyObj(Integer x) {
    if (x < 0) {
      throw new IllegalArgumentException("negative x!");
    }
    this.x = x;
  }
  
  public int getX() {
    return x;
  }
}

In a unit test, we can have

constraint violations in tests

Thu, 31 May 2018 00:00:00 +0000

I had some test code that looked something like

@Test public void a() {
  db.insert(0, "a");
  assertThat(db.query(1).getKey()).isEqualTo("a");
}

@Test public void b() {
  db.insert(0, "a");
  db.insert(1, "b");
  assertThat(db.query(2).getKey()).isEqualTo("b");
}

@Test public void c() {
  assertThat(db.query(3)).isNull();
}

There’s some stuff to like about this:

pretty straightforward which test is touching which records
pretty clear what you expect to see in the database.

There’s a bunch of coincidences here that make this work, though:

multiple thread test cases in java

Tue, 15 May 2018 00:00:00 +0000

Some of my day job lately has been trying to get a better handle on threads in Java. In particular, we had a really weird race condition resulting from multiple threads processing an input data stream and causing some unexpected conflicts in our data access layer.

To try to replicate the problem in tests, I started with something like this:

private static Runnable runWithDelay(String name, long millis) {
  return () -> {
    try {
      Thread.sleep(millis);
    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }
    System.out.println(name + " finished");
  };
}

This is just a Runnable that waits a while, and then prints that it finished. Then we can test with something like:

regex in java

Fri, 23 Feb 2018 13:41:22 +0000

I can never remember how to do java regexes with Pattern.

Pattern pattern = Pattern.compile("x_(yy|zz)");
String input = "x_yy";
boolean m = pattern.asPredicate().test(input);
System.out.println("matches: " + m);
Matcher matcher = pattern.matcher(input);
while (matcher.find()) {
  System.out.println("whole matched string: " + matcher.group(0));
  System.out.println("matched group: " + matcher.group(1));
}

I also learned a really cool thing IntelliJ can do with pattern matches!

![java regex]({{ site.baseurl }}/assets/java-regex.png)

guava RangeMap

Fri, 26 Jan 2018 09:52:41 +0000

We had some code like this:

NavigableMap<Double, Integer> PRIORITY_MULTIPLIERS = new TreeMap<Double, Integer>() {{ "{{ " }}
  put(LOW_THRESHOLD, 1);    // (20k..40k)  => 1
  put(MEDIUM_THRESHOLD, 2); // [40k..100k) => 2
  put(HIGH_THRESHOLD, 3);   // [100k..+∞)  => 3
}};

Integer getPriorityMultiplier(Double v) {
  if (v < LOW_THRESHOLD) {
    return 0;
  }
  return PRIORITY_MULTIPLIERS.floorEntry(v).getValue()
}

I replaced with a [RangeMap][RangeMap] that uses [Range][Range] instances as keys: [RangeMap]: https://google.github.io/guava/releases/16.0/api/docs/com/google/common/collect/RangeMap.html [Range]: https://google.github.io/guava/releases/16.0/api/docs/com/google/common/collect/Range.html

RangeMap<Double, Integer> PRIORITY_MULTIPLIERS = ImmutableRangeMap.<Double, Long>builder()
      .put(Range.lessThan(LOW_THRESHOLD), 0)
      .put(Range.closedOpen(LOW_THRESHOLD, MEDIUM_THRESHOLD), 1)
      .put(Range.closedOpen(MEDIUM_THRESHOLD, HIGH_THRESHOLD), 2)
      .put(Range.atLeast(HIGH_THRESHOLD), 3)
      .build();
Integer getPriorityMultiplier(Double v) {
  return rangePriorityMultipliers.get(v);
}

Advantages here being:

relational java explorations with sqlite3 vtab

Fri, 05 May 2017 20:10:28 +0000

One idea i have been mulling over lately is exposing a java codebase relationally, with queries like:

SELECT *
FROM classes c
JOIN annotations a
  ON a.class_id = c.id
WHERE a.name = 'MyAnnotationClass';

The idea here is that you could search a codebase by pretending that you had tables for a whole bunch of things like:

classes
annotations
variables
literals
methods

This would be a useful thing because instead of relying on an IDE like IntelliJ to do “find usages” operations, you could actually script those interactions. Admittedly, of course, Java has some pretty sophisticated reflection APIs to find this sort of stuff too, but it seems like exploring the structure could be much easier writing Java code to traverse those trees.

Ubuntu Hacks -- Flash, Java, e17

Mon, 20 Oct 2008 00:00:00 +0000

This must be about the 1 millionth time someone’s written a blog post about this, because I waded through a ton of them to get ones that worked. Which means someday, someone will be wading through 10 million websites trying to get their Flash, Java, and e17 working.

For the browser issues, I found this thread on the Ubuntu forums. In short, you download the first file listed(the Browser Install Script), untar it, and run it. It will download and install the 32-bit libraries, 32-bit browser, and plugins from the set of {Flash, Java, VLC}. The only trick then is rewriting your scripts to use firefox32 instead of just firefox.