??????????? “+” ????
?????????Java String?????1???? ?У??????????????final???Σ???? “+” ?????????????????л?????StringBuilder?????append??????????????????????????JVM?С?
????public class StringTest {
????public static void main(String[] args) {
????String str1 = "hello ";
????String str2 = "java";
????String str3 = str1 + str2 + "!";
????String str4 = new StringBuilder().append(str1).append(str2).append("!").toString();
????}
????}
???????? str3 ?? str4 ?????Ч???????????????????for????У???????? “+” ?????????????
????public class test {
????public static void main(String[] args) {
????run1();
????run2();
????}
????public static void run1() {
????long start = System.currentTimeMillis();
????String result = "";
????for (int i = 0; i
??????for???????? “+” ??StringBuilder????1?????????????????????￡?
????1????? “+” ?????????? 250ms??
????2?????StringBuilder?????????? 1ms??
????for???????? “+” ???????????????????run1??????????????
????2184951-aa3532a823ca11b5
????5 ~ 34 ?ж??for?????????????????????????????3????StringBuilder?????????????????????
????????????
????StringBuilder???????????char[]?????value?????????????append???????????????? new StringBuilder() ????????char[]?????????16?????append??17???????????????
????void expandCapacity(int minimumCapacity) {
????int newCapacity = value.length * 2 + 2;
????if (newCapacity - minimumCapacity
???????value??????????????????????????????expandCapacity(int minimumCapacity)??????value?????????????minimumCapacity = ?value???? + append????????????
????1???????????????????? + 2?????? + 2????????????????
????2???????????????????????????????? minimumCapacity ????????????С??
????3?????? System.arraycopy ???????value??????и????
?????????StringBuilder??????????????????????????????????char[]?????θ???????μ?????????
????????????????????????
????public class StringBuilderTest {
????public static void main(String[] args) {
????int sum = 0;
????final int capacity = 40000000;
????for (int i = 0; i
????????????append??????????????????????????￡?
????1??????????16?????????110ms??
????2???????40000000????????????????????????85ms??
??????????????????????????????????????
???????????
????1??StringBuilder????????????????????????С????????+2??char???飬???????char???鵽?????飬?????????????????GC???????
????2??StringBuilder??toString????????????char?????????
????public String toString() {
????// Create a copy?? don't share the array
????return new String(value?? 0?? count);
????}
????String????????У???????????С????char???飬????? System.arraycopy() ????StringBuilder??char??????????????StringBuilder??char????????????
????????StringBuilder
????public class StringBuilderHolder {
????private final StringBuilder sb;
????public StringBuilderHolder(int capacity) {
????sb = new StringBuilder(capacity);
????}
????public StringBuilder resetAndGet() {
????sb.setLength(0);
????return sb;
????}
????}
??????? sb.setLength(0) ?????????char?????????????????0??????char??????????????????????????????ThreadLocal?????StringBuilderHolder????÷?????￡?
????private static final ThreadLocal stringBuilder= new ThreadLocal() {
????@Override
????protected StringBuilderHolder initialValue() {
????return new StringBuilderHolder(256);
????}
????};
????StringBuilder sb = stringBuilder.get().resetAndGet();
????????????????????????????StringBuilder????????????????GC????????char????????β????б??????????????????????????????????????????????????????????
???????
??????????????StringBuilder???????????????????????????????????????????????￡???????????ThreadLocal????????StringBuilder??????