[wpiutil] Add ClassPreloader (#5365)

This provides an easy-to-use way to preload classes by name at startup to
avoid later delays due to lazy classloading.

wpiutil/src/main/java/edu/wpi/first/util/ClassPreloader.java

@@ -0,0 +1,119 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.util;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.NoSuchFileException;
+import java.nio.file.Paths;
+
+/**
+ * Loads classes by name. Can be used at any time, but is most commonly used to preload classes at
+ * the start of the program to avoid unpredictable delays due to lazy classloading later in program
+ * execution.
+ */
+public final class ClassPreloader {
+  private ClassPreloader() {}
+
+  /**
+   * Loads classes from an iterable.
+   *
+   * @param classNames iterable of class names
+   * @return Number of classes loaded.
+   */
+  public static int preload(Iterable<String> classNames) {
+    int count = 0;
+    for (String i : classNames) {
+      try {
+        Class.forName(i);
+        count++;
+      } catch (ClassNotFoundException e) {
+        System.out.println("Could not preload " + i);
+      }
+    }
+    return count;
+  }
+
+  /**
+   * Loads classes.
+   *
+   * @param classNames array of class names
+   * @return Number of classes loaded.
+   */
+  public static int preload(String... classNames) {
+    int count = 0;
+    for (String i : classNames) {
+      try {
+        Class.forName(i);
+        count++;
+      } catch (ClassNotFoundException e) {
+        System.out.println("Could not preload " + i);
+      }
+    }
+    return count;
+  }
+
+  /**
+   * Loads classes from a buffered reader. The input is expected to be one class name per line.
+   * Blank lines and lines starting with a semicolon are ignored.
+   *
+   * @param reader Reader
+   * @return Number of classes loaded.
+   */
+  public static int preload(BufferedReader reader) {
+    int count = 0;
+    try {
+      String line = reader.readLine();
+      while (line != null) {
+        if (!line.isEmpty() && !line.startsWith(";")) {
+          try {
+            Class.forName(line);
+            count++;
+          } catch (ClassNotFoundException e) {
+            System.out.println("Could not preload " + line);
+          }
+        }
+        line = reader.readLine();
+      }
+    } catch (IOException e) {
+      System.out.println("Error when reading preload file: " + e);
+    }
+    return count;
+  }
+
+  /**
+   * Loads classes from an input stream. The input is expected to be one class name per line. Blank
+   * lines and lines starting with a semicolon are ignored.
+   *
+   * @param stream input stream
+   * @return Number of classes loaded.
+   */
+  public static int preload(InputStream stream) {
+    return preload(new BufferedReader(new InputStreamReader(stream)));
+  }
+
+  /**
+   * Loads classes from a file. The input is expected to be one class name per line. Blank lines and
+   * lines starting with a semicolon are ignored.
+   *
+   * @param filename filename
+   * @return Number of classes loaded.
+   */
+  public static int preloadFile(String filename) {
+    try (BufferedReader reader =
+        Files.newBufferedReader(Paths.get(filename), StandardCharsets.UTF_8)) {
+      return preload(reader);
+    } catch (NoSuchFileException e) {
+      System.out.println("Could not open preload file " + filename + ": " + e);
+    } catch (IOException e) {
+      System.out.println("Could not close preload file " + filename + ": " + e);
+    }
+    return 0;
+  }
+}