深入jvm-1
初探字节码与jvm
public class Foo {
public static void main(String[] args) {
boolean flag = true;
if (flag) {
System.out.println("Hello, Java!");
}
if (true == flag) {
System.out.println("Hello, JVM!");
}
}
}
上面这段java代码,执行结果会是什么样的呢?问题会不会太弱鸡了?
编译
javac Foo.java # 先编译下它
执行
java -cp . Foo # 执行下试试咯
Hello, Java!
Hello, JVM!
它的字节码是什么样子的呢?
先来准备个小工具asmtools分析下
# cd ~/workspace/jvm/code-tools/
hg clone http://hg.openjdk.java.net/code-tools/asmtools && cd asmtools/build && ant
# 编译结果 ~/workspace/jvm/code-tools/asmtools-7.0-build/release/
字节码反编译为java汇编码(jasm码)
java -cp ~/workspace/jvm/code-tools/asmtools-7.0-build/release/lib/asmtools.jar org.openjdk.asmtools.jdis.Main Foo.class > Foo.jasm.orig
rm Foo.class # 不再需要了,删掉它
# cat Foo.jasm.orig
super public class Foo
version 52:0
{
public Method "<init>":"()V"
stack 1 locals 1
{
aload_0;
invokespecial Method java/lang/Object."<init>":"()V";
return;
}
public static Method main:"([Ljava/lang/String;)V"
stack 2 locals 2
{
iconst_1;
istore_1;
iload_1;
ifeq L14;
getstatic Field java/lang/System.out:"Ljava/io/PrintStream;";
ldc String "Hello, Java!";
invokevirtual Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
L14: stack_frame_type append;
locals_map int;
iconst_1;
iload_1;
if_icmpne L27;
getstatic Field java/lang/System.out:"Ljava/io/PrintStream;";
ldc String "Hello, JVM!";
invokevirtual Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
L27: stack_frame_type same;
return;
}
} // end Class Foo
修改下字节码反编译的结果java汇编码(jasm码)
awk 'NR==1, /iconst_1/{sub(/iconst_1/, "iconst_2")} 1' Foo.jasm.orig > Foo.jasm # 把第一个iconst_1改为iconst_2
# 文件编辑器也可以修改哈
把修改后的java汇编码(jasm码)编译成字节码
java -cp ~/workspace/jvm/code-tools/asmtools-7.0-build/release/lib/asmtools.jar org.openjdk.asmtools.jasm.Main Foo.jasm
执行从jasm编译出来的Foo.class试试看哦
此处暂停一下,想想看Foo.class能执行么?执行会输出什么呢?为什么呢?
java -cp . Foo
# Hello, Java!
为什么没输出"Hello, JVM!“呢?
- java/jvm把Boolean当作int来处理,flag变量被初始化为true,也就是1
- awk命令把flag初始值
iconst_1
改成了iconst_2
if(flag)
判断使用0值判断指令ifeq
,flag等于常数2为非0,所以输出了"Hello, Java!”if(true == flag)
判断使用整数相等判断指令if_icmpne
,flag = 2 !=iconst_1
(true),所以没有输出"Hello, JVM!"
修改后的jasm码
super public class Foo
version 52:0
{
public Method "<init>":"()V"
stack 1 locals 1
{
aload_0;
invokespecial Method java/lang/Object."<init>":"()V";
return;
}
public static Method main:"([Ljava/lang/String;)V"
stack 2 locals 2
{
iconst_2; # 把常数2推至栈顶
istore_1; # 将栈顶int型整数存入第二个本地变量; 也就是2
iload_1; # 将第2个int型本地变量推送至栈顶;也就是2
ifeq L14; # 当栈顶int型数值等于0时跳转至L14;2 当然不会等于0了,所以不跳转
getstatic Field java/lang/System.out:"Ljava/io/PrintStream;";
ldc String "Hello, Java!";
invokevirtual Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
L14: stack_frame_type append;
locals_map int;
iconst_1; # 把常数1推至栈顶
iload_1; # 将第2个int型本地变量推送至栈顶;也就是把2推送至栈顶
if_icmpne L27; 比较栈顶两个int型数值的大小,当结果不等时跳转至L27; 2 != 1 所以跳到了L27
getstatic Field java/lang/System.out:"Ljava/io/PrintStream;";
ldc String "Hello, JVM!";
invokevirtual Method java/io/PrintStream.println:"(Ljava/lang/String;)V";
L27: stack_frame_type same;
return;
}
} // end Class Foo