先看单线程
String withString = "";
long t0 = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
withString += "some string";
}
System.out.println("strings:" + (System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
StringBuffer buf = new StringBuffer();
for (int i = 0; i < 100000; i++) {
buf.append("some string");
}
System.out.println("Buffers : " + (System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
StringBuilder building = new StringBuilder();
for (int i = 0; i < 100000; i++) {
building.append("some string");
}
System.out.println("Builder : " + (System.currentTimeMillis() - t0));
输出
strings:51645
Buffers : 2
Builder : 2
可以看到string的很慢，Buffer和Builder差不多。
修改堆内存-Xmx10240m -Xmn10240m -XX:+PrintGCDetails，重新测试，buffer比builder慢了一点，string的连接操作伴随了大量的FullGC
因为string是不可变对象，拼接操作会伴随着大量char数组的扩容复制操作
strings:50391
Buffers : 3
Builder : 1
对测试方法做一点改动
long t0 = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
String withString = "some " + "string";
}
System.out.println("strings:" + (System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
StringBuffer buf = new StringBuffer();
buf.append("some ").append("string");
}
System.out.println("Buffers : " + (System.currentTimeMillis() - t0));
t0 = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
StringBuilder building = new StringBuilder();
building.append("some ").append("string");
}
System.out.println("Builder : " + (System.currentTimeMillis() - t0));
输出
strings:3
Buffers : 9
Builder : 8
这时发现string的拼接最快，因为String 对象的直接相加，JVM 会自动对其进行优化，也就是说 "some " + "string" 在编译期间会自动优化为 “some string”，直接一次性创建完成，所以效率肯定要高于StringBuffer和StringBuilder的 append 拼接。
但是要注意对于下面这种间接相加的操作，效率要比直接相加低，因为在编译器不会对引用变量进行优化。
String str1 = "Hello";
String str2 = "World";
String str3 = str1+str2;
测试一下
long t0 = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
String some = "some ";
String string = "string";
String withString = some + string;
}
System.out.println("strings:" + (System.currentTimeMillis() - t0));
输出
strings:17
在多线程下比较StringBuffer和StringBuilder
public class StringsPerf {
public static void main(String[] args) {
ThreadPoolExecutor executorService = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
//With Buffer
StringBuffer buffer = new StringBuffer();
for (int i = 0 ; i < 10; i++){
executorService.execute(new AppendableRunnable(buffer));
}
shutdownAndAwaitTermination(executorService);
System.out.println(" Thread Buffer : "+ AppendableRunnable.time);
//With Builder
AppendableRunnable.time = 0;
executorService = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
StringBuilder builder = new StringBuilder();
for (int i = 0 ; i < 10; i++){
executorService.execute(new AppendableRunnable(builder));
}
shutdownAndAwaitTermination(executorService);
System.out.println(" Thread Builder: "+ AppendableRunnable.time);
}
static void shutdownAndAwaitTermination(ExecutorService pool) {
pool.shutdown(); // code reduced from Official Javadoc for Executors
try {
if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
pool.shutdownNow();
if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
System.err.println("Pool did not terminate");
}
}
} catch (Exception e) {}
}
}
class AppendableRunnable<T extends Appendable> implements Runnable {
static long time = 0;
T appendable;
public AppendableRunnable(T appendable){
this.appendable = appendable;
}
@Override
public void run(){
long t0 = System.currentTimeMillis();
for (int j = 0 ; j < 10000 ; j++){
try {
appendable.append("some string");
} catch (IOException e) {}
}
time+=(System.currentTimeMillis() - t0);
}
}
可以看到StringBuffer明显要比StringBuilder慢
参考资料
https://stackoverflow.com/questions/355089/difference-between-stringbuilder-and-stringbuffer
https://mp.weixin.qq.com/s/dltB1Fzwdezs4PDN8Thx6w
