public class Main {
public static void main(String[] args) {
ArrayList<String> strList = new ArrayList<String>();
Type type = ((ParameterizedType)strList.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
System.out.println(type);
}
}
public class Main {
public static void main(String[] args) {
ArrayList<String> strList = new ArrayList<String>(){};
Type type = ((ParameterizedType)strList.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
System.out.println(type);
}
}这两个例子唯一的区别是后者的new ArrayList<String>(){}初始化strList的时候带了{}执行了赋初值，虽然语法层面没有什么区别，但是在编译之后的结果却完全不一样。而且执行的结果也完全不一样，
前者执行结果：
E后者执行结果：
class java.lang.String前者的编译结果：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: new #2 // class java/util/ArrayList
3: dup
4: invokespecial #3 // Method java/util/ArrayList."<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #4 // Method java/lang/Object.getClass:()Ljava/lang/Class;
12: invokevirtual #5 // Method java/lang/Class.getGenericSuperclass:()Ljava/lang/reflect/Type;
15: checkcast #6 // class java/lang/reflect/ParameterizedType
18: invokeinterface #7, 1 // InterfaceMethod java/lang/reflect/ParameterizedType.getActualTypeArguments:()[Ljava/lang/reflect/Type;
23: iconst_0
24: aaload
25: astore_2
26: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
29: aload_2
30: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
33: return这个逻辑很简单，就是简单的invokespecial了ArrayList的<init>()方法。
后者的编译结果：
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: new #2 // class Main$1
3: dup
4: invokespecial #3 // Method Main$1."<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #4 // Method java/lang/Object.getClass:()Ljava/lang/Class;
12: invokevirtual #5 // Method java/lang/Class.getGenericSuperclass:()Ljava/lang/reflect/Type;
15: checkcast #6 // class java/lang/reflect/ParameterizedType
18: invokeinterface #7, 1 // InterfaceMethod java/lang/reflect/ParameterizedType.getActualTypeArguments:()[Ljava/lang/reflect/Type;
23: iconst_0
24: aaload
25: astore_2
26: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
29: aload_2
30: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
33: return这里就奇怪了，加了{}之后竟然生成了内部类Main$1:
final class Main$1 extends java.util.ArrayList<java.lang.String>
...
{
Main$1();
descriptor: ()V
flags:
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/util/ArrayList."<init>":()V
4: return
LineNumberTable:
line 8: 0
}
Signature: #9 // Ljava/util/ArrayList<Ljava/lang/String;>;
...是java.util.ArrayList<java.lang.String>的子类，这也就解释了后者的执行结果为何可以解析到strList的泛型参数化类型是java.lang.String了，因为它的实际类型在JVM执行的时候清楚地被标记成了内部类Main$1这个java.util.ArrayList<java.lang.String>的子类。而前者的strList的泛型参数化类型已经被擦除掉了。