refactor(log):优化日志读取逻辑并修复潜在问题

- 使用 StringBuilder 替代 StringBuffer 提升性能- 采用 StandardCharsets.UTF_8 确保字符编码一致性
-优化行号判断逻辑，提升代码可读性- 移除冗余注释和无用代码片段-修复日志文件读取时的行数跳过逻辑
- 简化 LogResult 构造逻辑，移除不必要的注释块

xxl-job-core/src/main/java/com/xxl/job/core/log/XxlJobFileAppender.java

@@ -5,6 +5,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.io.*;
+import java.nio.charset.StandardCharsets;
 import java.text.SimpleDateFormat;
 import java.util.Date;
 
@@ -138,29 +139,35 @@ public class XxlJobFileAppender {
 	public static LogResult readLog(String logFileName, int fromLineNum){
 
 		// valid log file
-		if (logFileName==null || logFileName.trim().length()==0) {
+		if (logFileName==null || logFileName.trim().isEmpty()) {
             return new LogResult(fromLineNum, 0, "readLog fail, logFile not found", true);
 		}
 		File logFile = new File(logFileName);
-
 		if (!logFile.exists()) {
             return new LogResult(fromLineNum, 0, "readLog fail, logFile not exists", true);
 		}
 
 		// read file
-		StringBuffer logContentBuffer = new StringBuffer();
-		int toLineNum = 0;
+		StringBuilder logContentBuilder = new StringBuilder();
 		LineNumberReader reader = null;
+		int toLineNum = 0;
+		/*int readLineCount = 0;*/
 		try {
-			//reader = new LineNumberReader(new FileReader(logFile));
-			reader = new LineNumberReader(new InputStreamReader(new FileInputStream(logFile), "utf-8"));
+			reader = new LineNumberReader(new InputStreamReader(new FileInputStream(logFile), StandardCharsets.UTF_8));
 			String line = null;
-
 			while ((line = reader.readLine())!=null) {
+				// skip before lineNum
 				toLineNum = reader.getLineNumber();		// [from, to], start as 1
-				if (toLineNum >= fromLineNum) {
-					logContentBuffer.append(line).append("\n");
+				if (toLineNum < fromLineNum) {
+					continue;
 				}
+
+				// append log
+				logContentBuilder.append(line).append("\n");
+				// Limit return less than 1000 rows per query request	// todo
+				/*if(++readLineCount >= 5) {
+					break;
+				}*/
 			}
 		} catch (IOException e) {
 			logger.error(e.getMessage(), e);
@@ -175,15 +182,7 @@ public class XxlJobFileAppender {
 		}
 
 		// result
-		LogResult logResult = new LogResult(fromLineNum, toLineNum, logContentBuffer.toString(), false);
-		return logResult;
-
-		/*
-        // it will return the number of characters actually skipped
-        reader.skip(Long.MAX_VALUE);
-        int maxLineNum = reader.getLineNumber();
-        maxLineNum++;	// 最大行号
-        */
+        return new LogResult(fromLineNum, toLineNum, logContentBuilder.toString(), false);
 	}
 
 	/**