Java OOM 介紹與分析

EP1: What is OOM (2016/01/08)

1. What is OOM
2. GC
3. MAT introduction
4. MAT heapdump analyze
5. Practice

Outline

• What is OOM
• Why OOM
• Java Memory Model
• To be continued...

What is OOM

• Out Of My way?
• Out Of Mana?
• Out Of Memory!

Why OOM

• Try to allocate a very large memory.
• Memory leak. No sufficient memory available.
• Too many classes loaded.
• ??? God knows.

1.

byte[] huge = new byte[10_000_000_000L];

2.

/* Reproducible in Java 6 */
public class TestGC {
    private String largeString = new String(new byte[100000]);

    public String getString() {
        return largeString.substring(0, 2);
    }

    public static void main(String[] args) {
        List list = new ArrayList();
        for (int i = 0; i < 1000000; i++) {
            TestGC gc = new TestGC();
            list.add(gc.getString());
        }
    }
}

Java Memory Model

• Heap
  • Young generation
  • Old generation
• Non-heap