How many bytes does Java Object obj = new Object() occupy?
There is no sizeofoperator similar to C in Java . How to get the number of bytes actually occupied by an object?
method one:
A more convenient way is to observe by starting jvisualvm that comes with jdk.
Write a test class first
public class ObjectSizeInfo {
public static void main(String[] args) throws IOException {
ObjectSizeInfo sizes=new ObjectSizeInfo();
System.in.read();
}
}
As you can see, 1 object occupies 16 bytes
supplement
Since an Object object occupies 16 bytes, what are the contents stored in these 16 bytes?
- The first 8 bytes are the object header, also called markword, which records the various states of the object being locked (lock upgrade) and garbage collection related information.
- The next 4 bytes (below 4G heap memory; or less than 32G, with ClassPointer pointer compression enabled, otherwise 8 bytes ) is a pointer to the Class object to which the object belongs.
- The next 4 bytes are padding for 8-byte alignment.
Check whether pointer compression is turned on:
C:\Users\User>java -XX:+PrintFlagsFinal | find "UseCompressed"
bool UseCompressedClassPointers := true {lp64_product}
bool UseCompressedOops := true {lp64_product}UseCompressedClassPointers: compression options for class object pointers.
UseCompressedOops (oops-Ordinary Object Pointers): Common object pointer compression option.
Method Two
Use the jol (Java Object Layout) library provided by OpenJDK for observation.
Maven introduces dependencies:
<!-- https://mvnrepository.com/artifact/org.openjdk.jol/jol-core -->
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.14</version>
<scope>provided</scope>
</dependency>
public class ObjectSizeInfo {
public static void main(String[] args) throws IOException {
ObjectSizeInfo sizes=new ObjectSizeInfo();
String layout = ClassLayout.parseInstance(sizes).toPrintable();
System.out.println(layout);
}
}
Explanation: The instance object occupies 16 bytes.
Attached drawing: