StringBuffer 和 StringBuilder的区别

1. 一个线程安全，一个不安全
2. 不断调用toString方法，buffer不回重复new一个对象，但是builder会。

    @Test
    public void testBuffer() throws Exception {
        StringBuffer buffer = new StringBuffer("aaa");
        String s1 = buffer.toString();
        String s2 = buffer.toString();
        String s3 = buffer.toString();

        Field value = String.class.getDeclaredField("value");
        value.setAccessible(true);
        char[] o1 = (char[]) value.get(s1);
        char[] o2 = (char[]) value.get(s2);
        char[] o3 = (char[]) value.get(s3);
        System.out.println((Object) o1);
        System.out.println((Object) o2);
        System.out.println((Object) o3);
    }
//    [C@6996db8
//    [C@6996db8
//    [C@6996db8


    @Test
    public void testBuilder() throws Exception {
        StringBuilder buffer = new StringBuilder("aaa");
        String s1 = buffer.toString();
        String s2 = buffer.toString();
        String s3 = buffer.toString();

        Field value = String.class.getDeclaredField("value");
        value.setAccessible(true);
        char[] o1 = (char[]) value.get(s1);
        char[] o2 = (char[]) value.get(s2);
        char[] o3 = (char[]) value.get(s3);
        System.out.println((Object) o1);
        System.out.println((Object) o2);
        System.out.println((Object) o3);
    }
//    [C@6996db8
//    [C@1963006a
//    [C@7fbe847c