HowToMemoryLeakInJava

HowToMemoryLeakInJava.java

import java.util.ArrayList;
import java.util.List;

class Custom{
    private String name;
    Custom(String name){
        this.name = name;
    }
    protected void finalize(){
      //  System.out.println("gc'd");

    }
}
public class Test {
    @org.junit.Test
     public void test() throws InterruptedException {
        Runtime rt = Runtime.getRuntime();
        long freemem = rt.freeMemory();
        System.out.println(freemem / 1024 + " kb");
        for (int i = 0; i < 100; i++) {

        new Thread(new Runnable() {
            @Override
            public void run() {
                 List<Custom> list_customers= new ArrayList<>();
                while (true) {
                    list_customers.add( new Custom("john"));
                }

            }
        }).start();
    }


        while(true){
            Thread.sleep(5000);
            freemem = rt.freeMemory();
            System.out.println(freemem/1024 +" kb");
        }
    }
}
// -Xmx10m

/*
Exception in thread "Thread-11" java.lang.OutOfMemoryError: GC overhead limit exceeded
	at Test$1.run(Test.java:27)
	at java.lang.Thread.run(Thread.java:748)
Exception in thread "Thread-3" Exception in thread "Thread-4" java.lang.OutOfMemoryError: Java heap space
Exception in thread "Thread-13" java.lang.OutOfMemoryError: GC overhead limit exceeded
	at Test$1.run(Test.java:27)
	at java.lang.Thread.run(Thread.java:748)
java.lang.OutOfMemoryError: GC overhead limit exceeded
Exception in thread "Thread-31" Exception in thread "Thread-28" java.lang.OutOfMemoryError: GC overhead limit exceeded
Exception in thread "Thread-33" Exception in thread "Thread-20" Exception in thread "Thread-36" Exception in thread "Thread-43" java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
Exception in thread "Thread-0" Exception in thread "Thread-40" Exception in thread "Thread-26" Exception in thread "Thread-39" Exception in thread "Thread-30" java.lang.OutOfMemoryError: GC overhead limit exceeded
Exception in thread "Thread-9" Exception in thread "Thread-44" java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded


*/

VisualVM_Leak_Test.java

import java.util.ArrayList;
import java.util.List;

class Custom {
    String name;
    Custom another_relation;

    Custom(String name) {
        this.name = name;
    }

    protected void finalize() {
        //  System.out.println("gc'd");

    }
}

public class Test {
    @org.junit.Test
    public void test() throws InterruptedException {
        Runtime rt = Runtime.getRuntime();
        long freemem = rt.freeMemory();
        System.out.println(freemem / 1024 + " kb");
        List<Thread> t = new ArrayList<>();

        for (int i = 0; i < 50; i++) {

            t.add(new Thread(new Runnable() {
                @Override
                public void run() {
                    List<Custom> list_customers = new ArrayList<>();
                    while (true) {
                        try {
                            Thread.sleep(10);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        list_customers.add(new Custom("john"));
                    }

                }
            }));
            t.get(i).start();
        }


        while (true) {
            Thread.sleep(5000);
            freemem = rt.freeMemory();
            System.out.println(freemem / 1024 + " kb");
        }
    }
}
// -Xmx10m