shallow size就是对象本身占用内存的大小，比如String对象 public final class String implementsjava.io.Serializable, Comparable<String>, CharSequence { /**The value is used for character storage.*/ private final char value[]; /**The offset is the first index of the storage that is used.*/ private final int offset; /**The count is the number of characters in the String.*/ private final int count; /**Cache the hashcode for the string*/ private int hash;// Default to 0 。。。
32位的操作系统每个对象头占8个字节,有三个int属性，每个int占用4个字节 有一个char[]对象，这个对象无论有没有赋值，都有个null值，所以也占4个字节 所以每个String对象的shallow size，占用: 3*4 + 1*4 + 8 = 24个字节 很显然String如果有值，比如String str = new String(“123”);,那该String对象总共占用的字节是24+的， 这就要说retained size了 retained size：当该对象的内存释放后，GC总共能回收的内存，其实就是shallow size+reference object size 比如String str = new String(“a”); str对象的shallow size = 24 char[]对象的shallow size =8 char[]value的内存空间2*1=2 str对象的retained size = 24+8+2 = 34字节
