fix bug

hutool-core/src/main/java/cn/hutool/core/io/IoUtil.java

@@ -498,7 +498,7 @@ public class IoUtil extends NioUtil {
 	/**
 	 * 读取指定长度的byte数组，不关闭流
 	 *
-	 * @param in     {@link InputStream}，为null返回null
+	 * @param in     {@link InputStream}，为{@code null}返回{@code null}
 	 * @param length 长度，小于等于0返回空byte数组
 	 * @return bytes
 	 * @throws IORuntimeException IO异常
@@ -511,20 +511,9 @@ public class IoUtil extends NioUtil {
 			return new byte[0];
 		}
 
-		byte[] b = new byte[length];
-		int readLength;
-		try {
-			readLength = in.read(b);
-		} catch (IOException e) {
-			throw new IORuntimeException(e);
-		}
-		if (readLength > 0 && readLength < length) {
-			byte[] b2 = new byte[readLength];
-			System.arraycopy(b, 0, b2, 0, readLength);
-			return b2;
-		} else {
-			return b;
-		}
+		final FastByteArrayOutputStream out = new FastByteArrayOutputStream(length);
+		copy(in, out, DEFAULT_BUFFER_SIZE, length, null);
+		return out.toByteArray();
 	}
 
 	/**

hutool-core/src/test/java/cn/hutool/core/io/IoUtilTest.java

@@ -1,6 +1,7 @@
 package cn.hutool.core.io;
 
 import cn.hutool.core.io.resource.ResourceUtil;
+import cn.hutool.core.util.RandomUtil;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -10,14 +11,22 @@ import java.io.IOException;
 public class IoUtilTest {
 
 	@Test
-	public void readBytesTest(){
+	public void readBytesTest() {
 		final byte[] bytes = IoUtil.readBytes(ResourceUtil.getStream("hutool.jpg"));
 		Assert.assertEquals(22807, bytes.length);
 	}
 
 	@Test
-	public void readLinesTest(){
-		try(BufferedReader reader = ResourceUtil.getUtf8Reader("test_lines.csv");){
+	public void readBytesWithLengthTest() {
+		// 读取固定长度
+		final int limit = RandomUtil.randomInt(22807);
+		final byte[] bytes = IoUtil.readBytes(ResourceUtil.getStream("hutool.jpg"), limit);
+		Assert.assertEquals(limit, bytes.length);
+	}
+
+	@Test
+	public void readLinesTest() {
+		try (BufferedReader reader = ResourceUtil.getUtf8Reader("test_lines.csv");) {
 			IoUtil.readLines(reader, (LineHandler) Assert::assertNotNull);
 		} catch (IOException e) {
 			throw new IORuntimeException(e);