JOL 19 AL_LL

Java 评论

本篇文章基于V0.16

这个例子用于演示遍历可达性图。

我们除了可以观察对象内部，还可以观察到对象外部，比如，对象引用了哪些其他对象。
有很多种方式可以说明这一点，但是我们通过报表来演示一下。

在本例中，你可以清楚地看到ArrayList和LinkedList的shadow heap(即可达对象所占用的heap空间)之间的区别。

当Jol可以管理多个root时，它会分析每一个对象之间的可达性，从而避免重复计算。

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public class JOLSample_19_AL_LL {

    public static void main(String[] args) {
        out.println(VM.current().details());

        ArrayList<Integer> al = new ArrayList<>();
        LinkedList<Integer> ll = new LinkedList<>();

        for (int i = 0; i < 1000; i++) {
            Integer io = i; // box once
            al.add(io);
            ll.add(io);
        }

        al.trimToSize();

        PrintWriter pw = new PrintWriter(out);
        pw.println(GraphLayout.parseInstance(al).toFootprint());
        pw.println(GraphLayout.parseInstance(ll).toFootprint());
        pw.println(GraphLayout.parseInstance(al, ll).toFootprint());
        pw.close();
    }

}

运行结果

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# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

java.util.ArrayList@43556938d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1      4016      4016   [Ljava.lang.Object;
      1000        16     16000   java.lang.Integer
         1        24        24   java.util.ArrayList
      1002               20040   (total)


java.util.LinkedList@5579bb86d footprint:
     COUNT       AVG       SUM   DESCRIPTION
      1000        16     16000   java.lang.Integer
         1        32        32   java.util.LinkedList
      1000        24     24000   java.util.LinkedList$Node
      2001               40032   (total)


java.util.ArrayList@43556938d, java.util.LinkedList@5579bb86d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1      4016      4016   [Ljava.lang.Object;
      1000        16     16000   java.lang.Integer
         1        24        24   java.util.ArrayList
         1        32        32   java.util.LinkedList
      1000        24     24000   java.util.LinkedList$Node
      2003               44072   (total)

王明Coder

一个纯粹的技术开发者。