读写文件不会出现并发性？

[honey_fansy]

最近需要把日志写在文本中，我需要测试有没有并发性，但测出来没有并发性，按道理不同线程写入文件肯定有并发性的。
下面是代码：

package com.dragon.util;

public class WriteLogThread implements Runnable {

	private String fileName;
	private int index;
	
	public String getFileName() {
		return fileName;
	}

	public void setFileName(String fileName) {
		this.fileName = fileName;
	}

	public int getIndex() {
		return index;
	}

	public void setIndex(int index) {
		this.index = index;
	}

	@Override
	public void run() {
		for (int i = 0; i < 100; i ++) {
			try {
				WriteLogTool tool = WriteLogTool.instance;
				tool.writeToFileB(fileName, "[id" + this.index + "][name" + this.index + "][" + "value" + this.index + "]" + 
											"[id" + this.index + "][name" + this.index + "][" + "value" + this.index + "]\n" );
			} catch (Exception e) {
				e.printStackTrace();
			} 
		}
	}

}

package com.dragon.util;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.RandomAccessFile;

public enum WriteLogTool {
	
	instance();
	public void writeToFileA(String fileName, String record) throws IOException, InterruptedException {
		//RandomAccessFile randomAccessFile = new RandomAccessFile(new File(fileName), "rws");
		RandomAccessFile randomAccessFile = new RandomAccessFile(new File(fileName), "rw");
		long fileLength = randomAccessFile.length();
		randomAccessFile.seek(fileLength);
		randomAccessFile.writeBytes(record + "\n");
		randomAccessFile.close();
	}
	public void writeToFileB(String fileName, String record) throws IOException {
		FileWriter writer = new FileWriter(fileName, true);
		writer.write(record);
		writer.close();
	}

	public static String logToString(Object object) {
		String result = null;
		return result;
	}
	
	public static void main(String[] args) {
		String fileName = "D:\\1.txt";
		File file = new File(fileName);
		if (!file.exists()) {
			try {
				file.createNewFile();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		for (int i = 0; i < 100; i++) {
			WriteLogThread thread = new WriteLogThread();
			thread.setFileName(fileName);
			thread.setIndex(i);
			thread.run();
		}
	}
	
}

tool.writeToFileB(fileName, "[id" + this.index + "][name" + this.index + "][" + "value" + this.index + "]" +
"[id" + this.index + "][name" + this.index + "][" + "value" + this.index + "]\n" );
这一句我已经把字符串增加了好十几倍，让每一次写入文件的时间变长一点，但还是不会出现并发性的问题，为什么？