[BAEL-1951] Guide to FileChannel

core-java-io/src/test/java/com/baeldung/filechannel/FileChannelUnitTest.java

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+package com.baeldung.filechannel;
+
+import static org.junit.Assert.assertEquals;
+
+import java.io.ByteArrayOutputStream;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
+
+import org.junit.Test;
+
+public class FileChannelUnitTest {
+    @Test
+    public void givenFile_whenReadWithFileChannelUsingRandomAccessFile_thenCorrect() throws IOException {
+        String expected_value = "Hello world";
+        String file = "src/test/resources/test_read.in";
+        RandomAccessFile reader = new RandomAccessFile(file, "r");
+        FileChannel channel = reader.getChannel();
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+
+        int bufferSize = 1024;
+        if (bufferSize > channel.size()) {
+            bufferSize = (int) channel.size();
+        }
+        ByteBuffer buff = ByteBuffer.allocate(bufferSize);
+        int noOfBytesRead = channel.read(buff);
+
+        while (noOfBytesRead != -1) {
+            if (buff.hasArray()) {
+                out.write(buff.array(), 0, buff.position());
+                buff.clear();
+                noOfBytesRead = channel.read(buff);
+            }
+        }
+
+        assertEquals(expected_value, out.toString());
+        out.close();
+        channel.close();
+        reader.close();
+    }
+
+    @Test
+    public void givenFile_whenReadWithFileChannelUsingFileInputStream_thenCorrect() throws IOException {
+        String expected_value = "Hello world";
+        String file = "src/test/resources/test_read.in";
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        FileInputStream fin = new FileInputStream(file);
+        FileChannel channel = fin.getChannel();
+
+        int bufferSize = 1024;
+        if (bufferSize > channel.size()) {
+            bufferSize = (int) channel.size();
+        }
+        ByteBuffer buff = ByteBuffer.allocate(bufferSize);
+        int noOfBytesRead = channel.read(buff);
+
+        while (noOfBytesRead != -1) {
+            if (buff.hasArray()) {
+                out.write(buff.array(), 0, buff.position());
+                buff.clear();
+                noOfBytesRead = channel.read(buff);
+            }
+        }
+
+        assertEquals(expected_value, out.toString());
+        channel.close();
+        fin.close();
+    }
+
+    @Test
+    public void whenWriteWithFileChannelUsingRandomAccessFile_thenCorrect() throws IOException {
+        String input = "Hello world";
+        String file = "src/test/resources/test_write_using_filechannel.txt";
+
+        RandomAccessFile writer = new RandomAccessFile(file, "rw");
+        FileChannel channel = writer.getChannel();
+
+        ByteBuffer buff = ByteBuffer.wrap(input.getBytes());
+        while (buff.hasRemaining()) {
+            channel.write(buff);
+        }
+        channel.close();
+        writer.close();
+
+        // verify
+        RandomAccessFile reader = new RandomAccessFile(file, "r");
+        assertEquals(input, reader.readLine());
+        reader.close();
+    }
+
+    @Test
+    public void whenWriteWithFileChannelUsingFileOutputStream_thenCorrect() throws IOException {
+        String input = "Hello world";
+        String file = "src/test/resources/test_write_using_filechannel.txt";
+
+        FileOutputStream fout = new FileOutputStream(file);
+        FileChannel channel = fout.getChannel();
+
+        ByteBuffer buff = ByteBuffer.wrap(input.getBytes());
+        while (buff.hasRemaining()) {
+            channel.write(buff);
+        }
+
+        channel.close();
+        fout.close();
+
+        // verify
+        RandomAccessFile reader = new RandomAccessFile(file, "r");
+        assertEquals(input, reader.readLine());
+        reader.close();
+    }
+
+    @Test
+    public void givenFile_whenReadWithFileChannelGetPosition_thenCorrect() throws IOException {
+        long expected_value = 11;
+        String file = "src/test/resources/test_read.in";
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        RandomAccessFile reader = new RandomAccessFile(file, "r");
+        FileChannel channel = reader.getChannel();
+
+        int bufferSize = 1024;
+        if (bufferSize > channel.size()) {
+            bufferSize = (int) channel.size();
+        }
+        ByteBuffer buff = ByteBuffer.allocate(bufferSize);
+        int noOfBytesRead = channel.read(buff);
+
+        while (noOfBytesRead != -1) {
+            if (buff.hasArray()) {
+                out.write(buff.array(), 0, buff.position());
+                buff.clear();
+                noOfBytesRead = channel.read(buff);
+            }
+        }
+
+        assertEquals(expected_value, channel.position());
+
+        channel.position(4);
+        assertEquals(4, channel.position());
+
+        channel.close();
+        reader.close();
+    }
+
+    @Test
+    public void whenGetFileSize_thenCorrect() throws IOException {
+        long expectedSize = 11;
+        String file = "src/test/resources/test_read.in";
+        RandomAccessFile reader = new RandomAccessFile(file, "r");
+        FileChannel channel = reader.getChannel();
+
+        long imageFileSize = channel.size();
+        assertEquals(expectedSize, imageFileSize);
+
+        channel.close();
+        reader.close();
+    }
+
+    @Test
+    public void whenTruncateFile_thenCorrect() throws IOException {
+        long expectedSize = 5;
+        String input = "this is a test input";
+        String file = "src/test/resources/test_truncate.txt";
+
+        FileOutputStream fout = new FileOutputStream(file);
+        FileChannel channel = fout.getChannel();
+
+        ByteBuffer buff = ByteBuffer.wrap(input.getBytes());
+        channel.write(buff);
+        buff.flip();
+
+        channel = channel.truncate(5);
+        assertEquals(expectedSize, channel.size());
+
+        fout.close();
+        channel.close();
+    }
+}

core-java-io/src/test/resources/test_truncate.txt

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+this 

core-java-io/src/test/resources/test_write_using_filechannel.txt

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+Hello world