FileWriter增加方法，可选是否追加换行符（issue#3858@Github）

hutool-core/src/main/java/cn/hutool/core/io/file/FileWriter.java

@@ -237,13 +237,12 @@ public class FileWriter extends FileWrapper {
 						printNewLine(writer, lineSeparator);
 					}
 					writer.print(t);
-					if(appendLineSeparator){
-						printNewLine(writer, lineSeparator);
-					}
-
-					writer.flush();
 				}
 			}
+			if(appendLineSeparator){
+				printNewLine(writer, lineSeparator);
+			}
+			writer.flush();
 		}
 		return this.file;
 	}

hutool-core/src/test/java/cn/hutool/core/io/file/FileWriterTest.java

@@ -0,0 +1,24 @@
+package cn.hutool.core.io.file;
+
+import cn.hutool.core.collection.ListUtil;
+import cn.hutool.core.io.FileUtil;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+public class FileWriterTest {
+
+	@Test
+	@Disabled
+	void writeLinesAppendLineSeparatorTest() {
+		final FileWriter writer = FileWriter.create(FileUtil.file("d:/test/lines_append_line_separator.txt"));
+		writer.writeLines(ListUtil.of("aaa", "bbb", "ccc"), null, false, true);
+	}
+
+	@Test
+	@Disabled
+	void writeLinesTest() {
+		final FileWriter writer = FileWriter.create(FileUtil.file("d:/test/lines.txt"));
+		writer.writeLines(ListUtil.of("aaa", "bbb", "ccc"), null, false);
+	}
+
+}