Java中显式和隐式实例化String类的区别是什么
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我被告知像这样创建String实例
String s = new String("Don't do this"); // explicit
有一个性能问题,因为它为双引号词组“ Do n’t do this”创建了两个string实例,并为新的String()构造函数创建了一个实例!
今天我有时间自己进行测试,我创建了两个类:
public class String1 {
public static void main(String[] args) {
String s = new String("Hello");
System.out.println(s);
}
}
public class String2 {
public static void main(String[] args) {
String s = "Hello";
System.out.println(s);
}
}
这是javap的输出:
C:\jav>javap String1
Compiled from "String1.java"
public class String1 extends java.lang.Object{
public String1();
public static void main(java.lang.String[]);
}
C:\jav>javap String2
Compiled from "String2.java"
public class String2 extends java.lang.Object{
public String2();
public static void main(java.lang.String[]);
}
似乎它们是相同的,但是使用-c标志输出是不同的.
C:\jav>javap -c String1
Compiled from "String1.java"
public class String1 extends java.lang.Object{
public String1();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: new #2; //class java/lang/String
3: dup
4: ldc #3; //String Hello
6: invokespecial #4; //Method java/lang/String."<init>":(Ljava/lang/String;)V
9: astore_1
10: getstatic #5; //Field java/lang/System.out:Ljava/io/PrintStream;
13: aload_1
14: invokevirtual #6; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
17: return
}
C:\jav>javap -c String2
Compiled from "String2.java"
public class String2 extends java.lang.Object{
public String2();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2; //String Hello
2: astore_1
3: getstatic #3; //Field java/lang/System.out:Ljava/io/PrintStream;
6: aload_1
7: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
10: return
}
所以这是我的问题:)
首先,“ ldc”,astore_1等是什么?有没有描述这些的文件?
第二个javac真的不能弄清楚这两个句子是否相等吗?
解决方法:
Wikipedia has a very convenient summary of all the possible Java Bytecode instructions.此外,要获得完整图像,最好使用javap -v查看文件的全部内容,包括常量池:
Classfile /.../String1.class
Last modified 02/05/2013; size 458 bytes
MD5 checksum e3c355bf648c7441784ffc6b9765ba4d
Compiled from "String1.java"
public class String1
SourceFile: "String1.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #8.#17 // java/lang/Object."<init>":()V
#2 = Class #18 // java/lang/String
#3 = String #19 // Hello
#4 = Methodref #2.#20 // java/lang/String."<init>":(Ljava/l
ang/String;)V
#5 = Fieldref #21.#22 // java/lang/System.out:Ljava/io/Prin
tStream;
#6 = Methodref #23.#24 // java/io/PrintStream.println:(Ljava
/lang/String;)V
#7 = Class #25 // String1
#8 = Class #26 // java/lang/Object
#9 = Utf8 <init>
#10 = Utf8 ()V
#11 = Utf8 Code
#12 = Utf8 LineNumberTable
#13 = Utf8 main
#14 = Utf8 ([Ljava/lang/String;)V
#15 = Utf8 SourceFile
#16 = Utf8 String1.java
#17 = NameAndType #9:#10 // "<init>":()V
#18 = Utf8 java/lang/String
#19 = Utf8 Hello
#20 = NameAndType #9:#27 // "<init>":(Ljava/lang/String;)V
#21 = Class #28 // java/lang/System
#22 = NameAndType #29:#30 // out:Ljava/io/PrintStream;
#23 = Class #31 // java/io/PrintStream
#24 = NameAndType #32:#27 // println:(Ljava/lang/String;)V
#25 = Utf8 String1
#26 = Utf8 java/lang/Object
#27 = Utf8 (Ljava/lang/String;)V
#28 = Utf8 java/lang/System
#29 = Utf8 out
#30 = Utf8 Ljava/io/PrintStream;
#31 = Utf8 java/io/PrintStream
#32 = Utf8 println
{
public String1();
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>
":()V
4: return
LineNumberTable:
line 1: 0
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=2, args_size=1
0: new #2 // class java/lang/String
3: dup
4: ldc #3 // String Hello
6: invokespecial #4 // Method java/lang/String."<init>
":(Ljava/lang/String;)V
9: astore_1
10: getstatic #5 // Field java/lang/System.out:Ljav
a/io/PrintStream;
13: aload_1
14: invokevirtual #6 // Method java/io/PrintStream.prin
tln:(Ljava/lang/String;)V
17: return
LineNumberTable:
line 3: 0
line 4: 10
line 5: 17
}
现在,很清楚ldc从何处加载常数.
关于您的javac为什么不打扰这些优化的问题-主要是因为Java上完成的几乎所有优化几乎都推迟到运行时,在此运行其他编译器:JIT编译器,它将Java字节码编译为本地机器代码. javac确实做了一些努力来优化“常见”情况,但是这与抖动的攻击性相去甚远.
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