该程序将对象的状态转换为HTML字符串。
public class Test {
public static void main(String[] args) {
Address addr = new Address();
addr.setLine1("A straight line");
addr.setLine2("A curve");
addr.setCity("A Round City");
addr.setState("A Triangular State");
addr.setCountry("A Rectangle Country");
addr.setZip("123456");
@SuppressWarnings("unused")
String str;
int count = 1000;
for (int j = 0; j < 5; j++) {
double timeRich = System.nanoTime();
for (int i = 0; i < count; i++) {
str = AddressFormatter.formatRich(addr);
}
timeRich = System.nanoTime() - timeRich;
double timeFine = System.nanoTime();
for (int i = 0; i < count; i++) {
str = AddressFormatter.formatFine(addr);
}
timeFine = System.nanoTime() - timeFine;
double timePoor = System.nanoTime();
for (int i = 0; i < count; i++) {
str = AddressFormatter.formatPoor(addr);
}
timePoor = System.nanoTime() - timePoor;
System.out.println("Test cases: " + count);
System.out.println("Average time to format (SB Poor): " + (int)(timePoor/count) + " ns");
System.out.println("Average time to format (SB Fine): " + (int)(timeFine/count) + " ns");
System.out.println("Average time to format (String) : " + (int)(timeRich/count) + " ns");
System.out.println();
count *= 10;
}
System.out.println("***End of test***");
}
}
class Address {
private String line1;
private String line2;
private String city;
private String state;
private String country;
private String zip;
/**
* Default constructor.
*/
public Address() {}
public String getLine1() {
return line1;
}
public void setLine1(String line1) {
this.line1 = line1;
}
public String getLine2() {
return line2;
}
public void setLine2(String line2) {
this.line2 = line2;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getState() {
return state;
}
public void setState(String state) {
this.state = state;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public String getZip() {
return zip;
}
public void setZip(String zip) {
this.zip = zip;
}
}
class AddressFormatter {
// more readable than formatFine()
public static String formatPoor(Address obj) {
StringBuilder str = new StringBuilder();
str.append("<div class=\"address-wrapper\">\n");
str.append("\t<div class=\"addr-line\">" + obj.getLine1() + "</div>\n");
str.append("\t<div class=\"addr-line\">" + obj.getLine2() + "</div>\n");
str.append("\t<div class=\"addr-city\">" + obj.getCity() + "</div>\n");
str.append("\t<div class=\"addr-state\">" + obj.getState() + "</div>\n");
str.append("\t<div class=\"addr-country\">" + obj.getCountry() + "</div>\n");
str.append("\t<div class=\"addr-zip\">" + obj.getZip() + "</div>\n");
str.append("</div>\n");
return str.toString();
}
// grouping all constants, removing string concatenations
public static String formatFine(Address obj) {
StringBuilder str = new StringBuilder();
str.append("<div class=\"address-wrapper\">\n\t<div class=\"addr-line\">");
str.append(obj.getLine1());
str.append("</div>\n\t<div class=\"addr-line\">");
str.append(obj.getLine2());
str.append("</div>\n\t<div class=\"addr-city\">");
str.append(obj.getCity());
str.append("</div>\n\t<div class=\"addr-state\">");
str.append(obj.getState());
str.append("</div>\n\t<div class=\"addr-country\">");
str.append(obj.getCountry());
str.append("</div>\n\t<div class=\"addr-zip\">");
str.append(obj.getZip());
str.append("</div>\n</div>\n");
return str.toString();
}
public static String formatRich(Address obj) {
return "<div class=\"address-wrapper\">\n"
+ "\t<div class=\"addr-line\">" + obj.getLine1() + "</div>\n"
+ "\t<div class=\"addr-line\">" + obj.getLine2() + "</div>\n"
+ "\t<div class=\"addr-city\">" + obj.getCity() + "</div>\n"
+ "\t<div class=\"addr-state\">" + obj.getState() + "</div>\n"
+ "\t<div class=\"addr-country\">" + obj.getCountry() + "</div>\n"
+ "\t<div class=\"addr-zip\">" + obj.getZip() + "</div>\n"
+ "</div>\n";
}
}
在Eclipse中运行此程序时,我得到以下结果:
Test cases: 1000
Average time to format (SB Poor): 13513 ns
Average time to format (SB Fine): 7052 ns
Average time to format (String) : 14088 ns
Test cases: 10000
Average time to format (SB Poor): 3061 ns
Average time to format (SB Fine): 3290 ns
Average time to format (String) : 1618 ns
Test cases: 100000
Average time to format (SB Poor): 3486 ns
Average time to format (SB Fine): 1568 ns
Average time to format (String) : 589 ns
Test cases: 1000000
Average time to format (SB Poor): 616 ns
Average time to format (SB Fine): 547 ns
Average time to format (String) : 497 ns
Test cases: 10000000
Average time to format (SB Poor): 657 ns
Average time to format (SB Fine): 626 ns
Average time to format (String) : 191 ns
***End of test***
为什么String版本比StringBuilder版本更快?
为什么每次迭代后平均时间都在减少?
编辑: 我通过删除'StringBuilder'版本中的所有连接操作添加了另一个格式化函数(如一个答案所指出的)。
在第一次迭代中,'String'版本是最慢的。
在最后一次迭代中,'String'版本是最快的。
答案 0 :(得分:3)
问题的第二部分很简单:JVM识别重复执行并优化机器代码,这就是it's important to handle benchmarks carefully的原因。
以下是解释实施方面差异的原因:
你的&#34; StringBuilder
&#34;实现写得很差。您没有附加每个组件,而是执行字符串连接(为每个方法调用创建然后丢弃新的StringBuilder
) ,然后附加结果。如果您为每个元素正确使用了.append
,那么您可以看到更少的差异。
但是,现代Java编译器将一系列与+
的字符串连接转换为隐式StringBuilder
调用,以最大限度地减少对象创建。 Java编译器还合并连接的编译时字符串常量。因此,您的format2
方法也使用StringBuilder
,但有一个重要区别 - 所有行包装的相邻字符串常量都已合并。因此,即使您要修复format
方法,format2
也会更快,因为它会对更多固定内容进行分组。
答案 1 :(得分:1)
我预计formatRich
和formatFine
的字节代码是等价的,但事实并非如此。所以我试着得到两个等价的方法:
将StringBuilder方法重写为
public static String formatFine(Address obj) {
return new StringBuilder("<div class=\"address-wrapper\">\n\t<div class=\"addr-line\">")
.append(obj.getLine1())
.append("</div>\n\t<div class=\"addr-line\">")
.append(obj.getLine2())
.append("</div>\n\t<div class=\"addr-city\">")
.append(obj.getCity())
.append("</div>\n\t<div class=\"addr-state\">")
.append(obj.getState())
.append("</div>\n\t<div class=\"addr-country\">")
.append(obj.getCountry())
.append("</div>\n\t<div class=\"addr-zip\">")
.append(obj.getZip())
.append("</div>\n</div>\n").toString();
}
此方法相当于以下java字节代码:
public static String formatRich(Address obj) {
return "<div class=\"address-wrapper\">\n\t<div class=\"addr-line\">"
+ obj.getLine1()
+ "</div>\n\t<div class=\"addr-line\">"
+ obj.getLine2()
+ "</div>\n\t<div class=\"addr-city\">"
+ obj.getCity()
+ "</div>\n\t<div class=\"addr-state\">"
+ obj.getState()
+ "</div>\n\t<div class=\"addr-country\">"
+ obj.getCountry()
+ "</div>\n\t<div class=\"addr-zip\">"
+ obj.getZip()
+ "</div>\n</div>\n";
}
执行主程序会解析(在我的机器上):
...
Test cases: 10000000
Average time to format (SB Poor): 633 ns
Average time to format (SB Fine): 151 ns
Average time to format (String) : 152 ns
说明:
str.append
必须从堆栈加载str
。结果被推到堆栈上但从未使用过。StringBuilder
)重用已经在堆栈中的str.append
的结果str
和str.append
的结果)指向相同的堆位置,但我不知道这是否可以由编译器派生。似乎当前的优化级别无法对其进行优化。