JVM_09 类加载与字节码技术(字节码指令3)
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1.异常处理
try-catch
public class Demo3_11_1 {
public static void main(String[] args) {
int i = 0;
try {
i = 10;
} catch (Exception e) {
i = 20;
}
}
}
注意：为了抓住重点，下面的字节码省略了不重要的部分
字节码如下：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=1
0: iconst_0
1: istore_1
2: bipush 10
4: istore_1
5: goto 12
8: astore_2
9: bipush 20
11: istore_1
12: return
Exception table:
from to target type
2 5 8 Class java/lang/Exception
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
9 3 2 e Ljava/lang/Exception;
0 13 0 args [Ljava/lang/String;
2 11 1 i I
StackMapTable: ...
MethodParameters: ...
}
- 可以看到多出来一个 Exception table 的结构，[from, to) 是前闭后开的检测范围，一旦这个范围内的字节码执行出现异常，则通过 type 匹配异常类型，如果一致，进入 target 所指示行号
- 8 行的字节码指令 astore_2 是将异常对象引用存入局部变量表的 slot 2 位置
多个 single-catch 块的情况
public class Demo3_11_2 {
public static void main(String[] args) {
int i = 0;
try {
i = 10;
} catch (ArithmeticException e) {
i = 30;
} catch (NullPointerException e) {
i = 40;
} catch (Exception e) {
i = 50;
}
}
}
字节码如下：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=1
0: iconst_0
1: istore_1
2: bipush 10
4: istore_1
5: goto 26
8: astore_2
9: bipush 30
11: istore_1
12: goto 26
15: astore_2
16: bipush 40
18: istore_1
19: goto 26
22: astore_2
23: bipush 50
25: istore_1
26: return
Exception table:
from to target type
2 5 8 Class java/lang/ArithmeticException
2 5 15 Class java/lang/NullPointerException
2 5 22 Class java/lang/Exception
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
9 3 2 e Ljava/lang/ArithmeticException;
16 3 2 e Ljava/lang/NullPointerException;
23 3 2 e Ljava/lang/Exception;
0 27 0 args Ljava/lang/String;
2 25 1 i I
StackMapTable: ...
MethodParameters: ...
- 因为异常出现时，只能进入 Exception table 中一个分支，所以局部变量表 slot 2 位置被共用。
multi-catch 的情况
public class Demo3_11_3 {
public static void main(String[] args) {
try {
Method test = Demo3_11_3.class.getMethod("test");
test.invoke(null);
} catch (NoSuchMethodException | IllegalAccessException |
InvocationTargetException e) {
e.printStackTrace();
}
}
public static void test() {
System.out.println("ok");
}
}
字节码如下：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=2, args_size=1
0: ldc #2
2: ldc #3
4: iconst_0
5: anewarray #4
8: invokevirtual #5
11: astore_1
12: aload_1
13: aconst_null
14: iconst_0
15: anewarray #6
18: invokevirtual #7
21: pop
22: goto 30
25: astore_1
26: aload_1
27: invokevirtual #11 // e.printStackTrace:()V
30: return
Exception table:
from to target type
0 22 25 Class java/lang/NoSuchMethodException
0 22 25 Class java/lang/IllegalAccessException
0 22 25 Class java/lang/reflect/InvocationTargetException
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
12 10 1 test Ljava/lang/reflect/Method;
26 4 1 e Ljava/lang/ReflectiveOperationException;
0 31 0 args [Ljava/lang/String;
StackMapTable: ...
MethodParameters: ...
finally
public class Demo3_11_4 {
public static void main(String[] args) {
int i = 0;
try {
i = 10;
} catch (Exception e) {
i = 20;
} finally {
i = 30;
}
}
}
字节码如下：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=4, args_size=1
0: iconst_0
1: istore_1 // 0 -> i
2: bipush 10 // try --------------------------------------
4: istore_1 // 10 -> i |
5: bipush 30 // finally |
7: istore_1 // 30 -> i |
8: goto 27 // return -----------------------------------
11: astore_2 // catch Exceptin -> e ----------------------
12: bipush 20 // |
14: istore_1 // 20 -> i |
15: bipush 30 // finally |
17: istore_1 // 30 -> i |
18: goto 27 // return -----------------------------------
21: astore_3 // catch any -> slot 3 ----------------------
22: bipush 30 // finally |
24: istore_1 // 30 -> i |
25: aload_3 // <- slot 3 |
26: athrow // throw ------------------------------------
27: return
Exception table:
from to target type
2 5 11 Class java/lang/Exception
2 5 21 any // 剩余的异常类型，比如 Error
11 15 21 any // 剩余的异常类型，比如 Error
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
12 3 2 e Ljava/lang/Exception;
0 28 0 args [Ljava/lang/String;
2 26 1 i I
StackMapTable: ...
MethodParameters: ...
可以看到 finally 中的代码被复制了 3 份，分别放入 try 流程，catch 流程以及 catch 剩余的异常类型流程。
2.练习：finally面试题
finally 出现了 return
下面的题目输出什么？
public class Demo3_12_2 {
public static void main(String[] args) {
int result = test();
System.out.println(result);// 20
}
public static int test() {
try {
return 10;
} finally {
return 20;
}
}
}
字节码如下：
public static int test();
descriptor: ()I
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=0
0: bipush 10 // <- 10 放入栈顶
2: istore_0 // 10 -> slot 0 (从栈顶移除了)
3: bipush 20 // <- 20 放入栈顶
5: ireturn // 返回栈顶 int(20)
6: astore_1 // catch any -> slot 1
7: bipush 20 // <- 20 放入栈顶
9: ireturn // 返回栈顶 int(20)
Exception table:
from to target type
0 3 6 any
LineNumberTable: ...
StackMapTable: .
- 由于 finally 中的 ireturn 被插入了所有可能的流程，因此返回结果肯定以 finally 的为准
- 至于字节码中第 2 行，似乎没啥用，且留个伏笔，看下个例子。
- 跟上例中的 finally 相比，发现没有 athrow 了，这告诉我们：如果在 finally 中出现了 return，会吞掉异常，可以试一下下面。
.
public class Demo3_12_1 {
public static void main(String[] args) {
int result = test();
System.out.println(result);
}
public static int test() {
try {
int i = 1/0;
return 10;
} finally {
return 20;
}
}
}
finally 对返回值的影响
自己，下面的题目输出什么？
}
public class Demo3_12_2 {
public static void main(String[] args) {
int result = test();
System.out.println(result);// 20
}
public static int test() {
int i = 10;
try {
return i;
} finally {
i = 20;
}
}
}
字节码如下：
public static int test();
descriptor: ()I
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=3, args_size=0
0: bipush 10 // <- 10 放入栈顶
2: istore_0 // 10 -> i
3: iload_0 // <- i(10)
4: istore_1 // 10 -> slot 1，暂存至 slot 1，目的是为了固定返回值
5: bipush 20 // <- 20 放入栈顶
7: istore_0 // 20 -> i
8: iload_1 // <- slot 1(10) 载入 slot 1 暂存的值
9: ireturn // 返回栈顶的 int(10)
10: astore_2
11: bipush 20
13: istore_0
14: aload_2
15: athrow
Exception table:
from to target type
3 5 10 any
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
3 13 0 i I
StackMapTable: ...
3.synchronized关键字
public class Demo3_13 {
public static void main(String[] args) {
Object lock = new Object();
synchronized (lock) {
System.out.println("ok");
}
}
}
字节码如下：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=4, args_size=1
0: new #2 // new Object
3: dup
4: invokespecial #1 // invokespecial <init>:()V
7: astore_1 // lock引用 -> lock
8: aload_1 // <- lock （synchronized开始）
9: dup
10: astore_2 // lock引用 -> slot 2
11: monitorenter // monitorenter(lock引用)
12: getstatic #3 // <- System.out
15: ldc // <- "ok
17: invokevirtual #5 // invokevirtual println:
(Ljava/lang/String;)V
20: aload_2 // <- slot 2(lock引用)
21: monitorexit // monitorexit(lock引用)
22: goto 30
25: astore_3 // any -> slot 3
26: aload_2 // <- slot 2(lock引用)
27: monitorexit // monitorexit(lock引用)
28: aload_3
29: athrow
30: return
Exception table:
from to target type
12 22 25 any
25 28 25 any
LineNumberTable: ...
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 args [Ljava/lang/String;
8 23 1 lock Ljava/lang/Object;
StackMapTable: ...
MethodParameters: ...
注意：方法级别的 synchronized 不会在字节码指令中有所体现
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