📄 9% (0.09x) speedup for Util.readFile in client/src/com/aerospike/client/util/Util.java

⏱️ Runtime : 24.7 milliseconds → 22.6 milliseconds (best of 5 runs)

📝 Explanation and details

Primary benefit — runtime improved by ~9% (24.7 ms → 22.6 ms). The optimized version is faster because it replaces a Java-level manual read loop with the platform-provided FileInputStream.readAllBytes() call, yielding a measurable reduction in per-call overhead and fewer small reads/copies.

What changed

• Replaced the manual loop that repeatedly called FileInputStream.read(bytes, pos, len) with a single call to FileInputStream.readAllBytes().
• Removed manual buffer management (pos/len) and pre-calculation of byte[] length.
• Requires Java 9+ (readAllBytes is available on FileInputStream / InputStream in Java 9+).

Why this speeds up execution

• Eliminates per-iteration Java overhead: the manual loop performed many calls into read(...) and did bookkeeping (pos, len, loop checks). Each call carries JVM/bytecode overhead and JNI/native transitions. readAllBytes lets the JDK use an implementation optimized to minimize those transitions and to read in larger chunks.
• Fewer intermediate copies and fewer read calls: the JDK implementation can grow/allocate buffers and use larger native reads, reducing the number of System.arraycopy-like operations compared with many small reads into the same target buffer.
• Simpler code path: less branching and fewer Java-level instructions result in reduced CPU cycles for the same total bytes read.

Additional practical benefits

• Fixes a latent bug in the original implementation: the manual loop did not check for read(...) returning -1 (EOF), which can lead to incorrect behavior. The readAllBytes approach handles EOF correctly.
• Tests (small, medium, large files) pass and show correctness across file sizes; medium/large files generally see the biggest gains because they expose the cost of repeated reads/copies in the original loop.

Notes and trade-offs

• Dependency: relies on Java 9+ API. If the project must support older JVMs, this change is not usable.
• Memory characteristics are similar: both produce a single byte[] of the file contents (so large files still consume equivalent heap).
• Line profiler output shows more time attributed to the FileInputStream setup line in the optimized trace; this is a profiling-artifact concentration point, but overall wall-clock runtime decreased. The net runtime win shows the optimization reduced the overall work despite profiler attribution differences.

When to expect the biggest wins

• Hot paths that repeatedly read whole files, and medium-to-large files where the cost of multiple small reads and Java-level loop overhead is significant.

In short: switching to readAllBytes reduces per-call overhead, lets the JDK perform larger/optimized reads and corrects EOF handling — producing the measured ~9% runtime improvement while keeping behavior correct for the tested workloads.

✅ Correctness verification report:

package com.aerospike.client.util;

import org.junit.Before;
import org.junit.Test;
import org.junit.Rule;
import org.junit.rules.TemporaryFolder;
import static org.junit.Assert.*;

import java.io.File;
import java.io.FileOutputStream;

import com.aerospike.client.util.Util;

public class UtilTest {

    @Rule
    public TemporaryFolder tempFolder = new TemporaryFolder();

    private File smallFile;
    private byte[] expectedSmall;
    private File mediumFile;
    private byte[] expectedMedium;
    private File largeFile;
    private byte[] expectedLarge;

    @Before
    public void setUp() throws Exception {
        smallFile = tempFolder.newFile("small.txt");
        expectedSmall = "Hello, World!".getBytes();
        try (FileOutputStream out = new FileOutputStream(smallFile)) {
            out.write(expectedSmall);
        }

        mediumFile = tempFolder.newFile("medium.dat");
        expectedMedium = new byte[256 * 1024];
        for (int i = 0; i < expectedMedium.length; i++) {
            expectedMedium[i] = (byte) (i % 251);
        }
        try (FileOutputStream out = new FileOutputStream(mediumFile)) {
            out.write(expectedMedium);
        }

        largeFile = tempFolder.newFile("large.dat");
        expectedLarge = new byte[1024 * 1024];
        for (int i = 0; i < expectedLarge.length; i++) {
            expectedLarge[i] = (byte) (i % 256);
        }
        try (FileOutputStream out = new FileOutputStream(largeFile)) {
            out.write(expectedLarge);
        }
    }

    @Test
    public void testReadSmallFile() throws Exception {
        byte[] result = Util.readFile(smallFile);
        assertArrayEquals(expectedSmall, result);
    }

    @Test
    public void testReadMediumFile() throws Exception {
        byte[] result = Util.readFile(mediumFile);
        assertArrayEquals(expectedMedium, result);
    }

    @Test
    public void testReadLargeFile() throws Exception {
        byte[] result = Util.readFile(largeFile);
        assertArrayEquals(expectedLarge, result);
    }
}

To edit these changes git checkout codeflash/optimize-Util.readFile-mlsoak0f and push.