jre-utils

A library to help with JRE stuff like shutdownhooks or reading resources.

Collections

This package contains several collections I've come across and used in some programs.

DataQueue

A synchronized, size-limited FIFO-queue.

SizeLimitedHashMap

A normal HashMap, but limited in size. If there is a size-overflow, the oldest value will be discarded.

DataMap

A synchronized, size-limited HashMap.

DataTable

This is a data-structure that holds arbitrary objects in a DataQueue and you may or may not add one or many indexes, which will generate a DataMap for each index, using the given keySupplier to generate the key for each entry.

When you add a new element, delete an element or clear the list, all indexes will be automatically equally affected.

The indexes use minimal memory, since the references to the corresponding values is shared.

DataTable<String> dt = new DataTable<>(String.class, 10);
dt.addIndex("index1", e -> e);
dt.addIndex("index2", e -> e);
dt.add("test");
assertThat(dt.get("index1", "test")).isEqualTo("test");
assertThat(dt.get("index2", "test")).isEqualTo("test");

DoubleBufferedFile

This is a data-structure that is thread-safe and uses two files to persist itself. This way you have a file left, if something happens during persisting.

DoubleBufferedFile dbf = new DoubleBufferedFile(Path.of("new"), "txt");
dbf.write(w -> w.write("test_old"));
dbf.write(w -> w.write("test_new"));
String value = dbf.read() // is equal to "test_new"
dbf.delete();

The above example generates two files: new1.txt and new2.txt and the accessors always give the correct file-handle to read or write-to.

DateUtils

Offers some often used date-time conversions including UTC and ISO8601.

ForName

This helps you to load and instantiate classes that haven't already been loaded using a given class-loader. This is of great help to strip boilerplate code when doing stuff like plugin-systems or the like.

Resources

Helps reading and searching for resource files.

The special thing about these is, that they work in JAR-files AND server-deployments alike.

ShutdownHook

If you'd like some code running before the Java-VM shuts down, then this is the way to go. I use it for shutting down the EntityManagerFactory in some programs as gracefully as it gets.

Example

Shut down the executor-service gracefully at the end of your program.

ShutdownHook.register(() -> {
    executorService.shutdown();
    try {
        if (!executorService.awaitTermination(1, TimeUnit.SECONDS))
            executorService.shutdownNow();
    } catch (InterruptedException e) {
        executorService.shutdownNow();
    }
});

Reflecting

Reflection utilities. If you'd like to scan for a specific class-type within an object tree, then this could be of help. Same if you'd like to get the instance of such a field within an instance, but get it by path-name.

Example

// getFieldByPath
TestClass tc = new TestClass();
MyType mt = Reflecting.getFieldByPath("subClass.myType", tc, ContainsMyType.class);
mt.setName("blubb");
assertThat(tc.getSubClass().getMyType().getName()).isEqualTo("blubb");
assertThat(tc.getMyType().getName()).isNull();

// getPathsOf
List<String> results = Reflecting.getPathsOf(TestClass.class, MyType.class, ContainsMyType.class);
assertThat(results).contains("myType");
assertThat(results).contains("subClass.myType");

Exceptions

This utility-class contains swallow and swallowReturning which helps you to swallow a specific exception (checked or unchecked) silently.

Example

Before

@Test
public void BeforeTest() {
    try {
        // Some method you don't have control over throwing an exception.
        throw new IllegalArgumentException();
    } catch (IllegalArgumentException e) {
        // NOOP.
    }
}

The problem with this is that it's very verbose, possibly destroys the good visual style you're currently running in your library, or that your linter complains about NOOP catches.

After

Exceptions.swallow(() -> {
    throw new IllegalArgumentException();
}, IllegalArgumentException.class);

Obviously, you can use swallowReturning for methods that return a value and throw a checked or unchecked exception.

The Exception parameter is a varArg, so you may specify more than one Exception here.