Java 装箱与拆箱详解
前言：
要理解装箱和拆箱的概念，就要理解Java数据类型
装箱：把基本类型用它们相应的引用类型包装起来，使其具有对象的性质。int包装成Integer、float包装成Float
拆箱：和装箱相反，将引用类型的对象简化成值类型的数据
Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i)) int b = new Integer(100); 这是自动拆箱
看下面一段代码
m1
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m11();
dt.m12();
}
public void m11() {
Integer a = new Integer(100);
Integer b = 100;
System.out.println("m11 result " + (a == b));
}
public void m12() {
Integer a = new Integer(128);
Integer b = 128;
System.out.println("m12 result " + (a == b));
}
}
打印结果是什么？
m11 result false m12 result false
“==”比较的是地址，而a和b两个对象的地址不同，即是两个对象，所以都是false
通过javap解析字节码，内容如下
public void m11(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."":(I)V 9: astore_1 10: bipush 100 12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 15: astore_2 16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 19: new #59; //class java/lang/StringBuilder 22: dup 23: ldc #61; //String m11 result 25: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 28: aload_1 29: aload_2 30: if_acmpne 37 33: iconst_1 34: goto 38 37: iconst_0 38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 47: return public void m12(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer." ":(I)V 10: astore_1 11: sipush 128 14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 17: astore_2 18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 21: new #59; //class java/lang/StringBuilder 24: dup 25: ldc #82; //String m12 result 27: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 30: aload_1 31: aload_2 32: if_acmpne 39 35: iconst_1 36: goto 40 39: iconst_0 40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 49: return
m2
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m21();
dt.m22();
}
public void m21() {
Integer a = new Integer(100);
Integer b = new Integer(100);
System.out.println("m21 result " + (a == b));
}
public void m22() {
Integer a = new Integer(128);
Integer b = new Integer(128);
System.out.println("m22 result " + (a == b));
}
}
打印结果是
m21 result false m22 result false
a和b仍是两个对象
javap解析内容
public void m21(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."":(I)V 9: astore_1 10: new #44; //class java/lang/Integer 13: dup 14: bipush 100 16: invokespecial #46; //Method java/lang/Integer." ":(I)V 19: astore_2 20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 23: new #59; //class java/lang/StringBuilder 26: dup 27: ldc #84; //String m21 result 29: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 32: aload_1 33: aload_2 34: if_acmpne 41 37: iconst_1 38: goto 42 41: iconst_0 42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 51: return public void m22(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer." ":(I)V 10: astore_1 11: new #44; //class java/lang/Integer 14: dup 15: sipush 128 18: invokespecial #46; //Method java/lang/Integer." ":(I)V 21: astore_2 22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 25: new #59; //class java/lang/StringBuilder 28: dup 29: ldc #86; //String m22 result 31: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 34: aload_1 35: aload_2 36: if_acmpne 43 39: iconst_1 40: goto 44 43: iconst_0 44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 53: return
m3
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m31();
dt.m32();
}
public void m31() {
Integer a = 100;
Integer b = 100;
System.out.println("m31 result " + (a == b));
}
public void m32() {
Integer a = 128;
Integer b = 128;
System.out.println("m32 result " + (a == b));
}
}
打印结果
m31 result true m32 result false
为什么有第一个是true，第二个是false呢？观察javap解析的数据
javap解析内容
public void m31(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #88; //String m31 result 21: invokespecial #63; //Method java/lang/StringBuilder."":(Ljava/la ng/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 43: return public void m32(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #90; //String m32 result 23: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 45: return
m4
public class DataType {
public static void main(String args[]) {
DataType dt = new DataType();
dt.m41();
dt.m42();
}
public void m41() {
Integer a = Integer.valueOf(100);
Integer b = 100;
System.out.println("m41 result " + (a == b));
}
public void m42() {
Integer a = Integer.valueOf(128);
Integer b = 128;
System.out.println("m42 result " + (a == b));
}
}
打印结果
m41 result true m42 result false
javap解析内容
public void m41(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #92; //String m41 result 21: invokespecial #63; //Method java/lang/StringBuilder."":(Ljava/la ng/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 43: return public void m42(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #94; //String m42 result 23: invokespecial #63; //Method java/lang/StringBuilder." ":(Ljava/la ng/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 45: return }
分析
javap是Java自带的一个工具，可以反编译，也可以查看Java编译器生成的字节码(上面代码只使用了javap -c DataType)，是分析代码的一个好工具，具体怎么使用请Google一下
先看一下m4，为什么运行结果中出现了“true”呢，true说明a、b是同一个对象。
但a对象是调用Integer.valueOf()生成的，b是通过自动装箱生成的对象，为什么会是同一个对象呢？再看一下字节码吧，毕竟Java程序是依靠虚拟机运行字节码实现的。
m41这个方法只适用了一次valueOf()，但字节码中出现了两次，说明自动装箱时也调用了valueOf()。
下面是valueOf()具体实现
/**
* Returns a Integer instance representing the specified
* int value.
* If a new Integer instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Integer(int)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param i an int value.
* @return a Integer instance representing i.
* @since 1.5
*/
public static Integer valueOf(int i) {
final int offset = 128;
if (i >= -128 && i <= 127) { // must cache
return IntegerCache.cache[i + offset];
}
return new Integer(i);
}
在【-128，127】之间的数字，valueOf返回的是缓存中的对象，所以两次调用返回的是同一个对象。
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