一、优化过程：

问题：所有文件读取慢。
优化1、之前：一个字节一个字节的读取。之后每次读取8*1024个字节。效率提高100倍。
问题：大文件读取慢。
优化2：之前内容用LinkedList，之后用ArrayList。LinkedList差寻慢，不是从头查就是从尾查
问题：读取大文件oom
原因：之前先一次性读取到StringBuilder（StringBuilder一开始被分配在young区，15次ygc后晋升到old区，晋升到old区的StringBuilder依然不断增大，直至oom）。已后期验证同样的数据StringBuilder将近比ArrayList大一倍。
优化3：之前先一次性读取到StringBuilder。之后每次读取8*1024个字节到StringBuilder（利用了young区的oom），同时加到ArrayList。

二、demo

package d;import lombok.extern.slf4j.Slf4j;import java.io.*;
import java.nio.charset.Charset;
import java.util.*;
import java.util.concurrent.CountDownLatch;/*** @author sunxuejian* @date 2020/6/23*/
@Slf4j
public class Test_IO {public final static String PARAMETERIZED_FILE = "parameterized.properties";protected static Charset charset = Charset.forName("utf-8");static {InputStream is = AbstractAsciiFileAccess.class.getClassLoader().getResourceAsStream(PARAMETERIZED_FILE);Properties properties = new Properties();try {properties.load(is);String encoding = properties.getProperty("file_encoding", "utf-8");System.out.println("===========>encoding" + encoding);if (!"".equals(encoding = encoding.trim())) {charset = Charset.forName(encoding);}} catch (IOException e) {e.printStackTrace();}}protected RandomAccessFile randomAccessFile;protected String separator;protected String readMode = "r";protected boolean enCycle;//是否回去读protected long lineCount;protected void setLineCount(File file) {LineNumberReader rf = null;try {rf = new LineNumberReader(new FileReader(file));if (rf != null) {rf.skip(file.length());this.lineCount = rf.getLineNumber() + 1;}} catch (IOException e) {log.error(e.getMessage(), e);} finally {if (rf != null) {try {rf.close();} catch (IOException e) {log.error(e.getMessage(), e);}}}}public String readLine() {try {return this.nextLine();} catch (IOException e) {e.printStackTrace();}return null;}private volatile Boolean isReaded = false;private int readLineCount = 0;List<String> lines = new ArrayList();private synchronized String nextLine() throws IOException {if(!isReaded) {StringBuilder builder = new StringBuilder();byte[] b = new byte[80 * 1024];int i;while(true) {i=randomAccessFile.read(b);if (i == -1) {lines.add(builder.toString());break;}builder.append(new String(b, 0, i, charset));List<String> lineList = Arrays.asList(builder.toString().split(System.getProperty("line.separator")));int length = lineList.size();if (length == 1){continue;}for (int j=0; j<length-1; j++){lines.add(lineList.get(j));}builder = new StringBuilder(lineList.get(length-1));}isReaded = true;}if(readLineCount >= lineCount) {if(enCycle){readLineCount=0;}else {return null;}}String s = lines.get(readLineCount++);
//        System.out.println("====>" + s);return s;}public Test_IO(String filepath) {this(filepath, null, false);}public Test_IO(String filepath, String separator, boolean enCycle) {try {File file = new File(filepath);if (file.exists() && file.isFile()) {randomAccessFile = new RandomAccessFile(file, readMode);} else {file = new File(AsciiFileAccessCircularly_type2.class.getClassLoader().getResource(filepath).getFile());randomAccessFile = new RandomAccessFile(file, readMode);}this.separator = separator;this.enCycle = enCycle;this.setLineCount(file);} catch (FileNotFoundException e) {e.printStackTrace();}}static Test_IO afc = new Test_IO("F:\\test-io\\random.txt");public static void main(String[] args) throws InterruptedException {CountDownLatch countDownLatch = new CountDownLatch(10);long x = System.currentTimeMillis();for(int i=0; i<10; i++){new Thread(new Runnable() {@Overridepublic void run() {while (true) {String s = afc.readLine();
//                        System.out.println("==>" + s);if(s == null) {break;}}countDownLatch.countDown();}},"thread" + i).start();}countDownLatch.await();long y = System.currentTimeMillis();System.out.println("耗时：" + (y-x));}}