Java memory analyzer

Origin

When I optimize a mysql pagination interface , I want ot use Redis cache to store some result to speed it up. In order to estimate how much memory I will spend, I have to do some memory estimation.

Solution

This solution comes from this stackoverflow question.

Steps

Write the following source:

package memory;

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

Add the following line to your MANIFEST.MF:

Premain-Class: memory.ObjectSizeFetcher

Use getObjectSize:

package memory;

public class C {
    private int x;
    private int y;

    public static void main(String [] args) {
        System.out.println(
        ObjectSizeFetcher.getObjectSize(new C()));
    }
}

Build your jar as you have done before or use:

jar cvfm ObjectSizeFetcherAgent.jar MANIFEST.MF input_file`

Invoke with:

java -javaagent:ObjectSizeFetcherAgent.jar memory.C

Runnable source can be found here

Other solution

Limitation

As comments of above stackoverflow question pointed out, above solution has some limits: it just count reference size for Object not the size in heap.
For example, for the following class, program all give 16:

private static class JustString {
    private String a = "test";
}

private static class ByteArray {
    private byte[] a = new byte[128];
}

private static class JustObject {
    private Object o = new Object();
}

private static class JustList {
    private List<A> l = new ArrayList<>();
}

So we move to more productive solution

Solution from netty

When skim the source of netty, I find a thread pool executor named MemoryAwareThreadPoolExecutor. Interested in this class, I find out the source and read the following doc:

When a task (i.e. Runnable) is submitted, MemoryAwareThreadPoolExecutor calls ObjectSizeEstimator.estimateSize(Object) to get the estimated size of the task in bytes to calculate the amount of memory occupied by the unprocessed tasks.
If the total size of the unprocessed tasks exceeds either per-Channel or per-Executor threshold, any further execute(Runnable) call will block until the tasks in the queue are processed so that the total size goes under the threshold.

This executor block job submission when exceed memory limit and memory analyzer is a crutial part. We can find a implementation for ObjectSizeEstimator in source
— DefaultObjectSizeEstimator, and we will analyze how it implemented.
First, it store the size of primitives:

public DefaultObjectSizeEstimator() {
    class2size.put(boolean.class, 4); // Probably an integer.
    class2size.put(byte.class, 1);
    class2size.put(char.class, 2);
    class2size.put(int.class, 4);
    class2size.put(short.class, 2);
    class2size.put(long.class, 8);
    class2size.put(float.class, 4);
    class2size.put(double.class, 8);
    class2size.put(void.class, 0);
}

Then, it analyze this class recursively for every field:

visitedClasses.add(clazz);

int answer = 8; // Basic overhead.
for (Class<?> c = clazz; c != null; c = c.getSuperclass()) {
    Field[] fields = c.getDeclaredFields();
    for (Field f : fields) {
        if ((f.getModifiers() & Modifier.STATIC) != 0) {
            // Ignore static fields.
            continue;
        }

        answer += estimateSize(f.getType(), visitedClasses);
    }
}

visitedClasses.remove(clazz);

And then align to 8 byte:

// Some alignment.
answer = align(answer);

Finally some special handler for some class:

...
} else if (o instanceof byte[]) {
    answer += ((byte[]) o).length;
} else if (o instanceof ByteBuffer) {
    answer += ((ByteBuffer) o).remaining();
} else if (o instanceof CharSequence) {
    answer += ((CharSequence) o).length() << 1;
} else if (o instanceof Iterable<?>) {
    for (Object m : (Iterable<?>) o) {
        answer += estimateSize(m);
    }
}

What should be noticed is this class is designed for netty using which omit many special cases( For example, it omit the handle process for a int array.) and not suitable for common usage.

But most of the code can be used to customize your version of memory analyzer.

Online source code of DefaultObjectSizeEstimator

Ref

• javaagent option
• tutorials about javaagent
• create a jar use command line

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