stringBuilder append方法

package org.zyl.test;

public class Test1 {

 public static void main(String[] args) throws InterruptedException {
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < 10; i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    for (int j = 0; j < 100000; j++){
                    	synchronized (stringBuilder) {
                    		stringBuilder.append("a");
						}
                    }
                }
            }).start();
        }

        Thread.sleep(100);
        System.out.println(stringBuilder.length());
    }

}
此时synchronized会出现锁不住stringBuilder的现象，导致最终会出现stringBuilder中数据出现不够，当睡眠时间增长以后会解决，但是为什么会锁不住对象我还是没有弄明白