为什么Java不释放内存

Question

I'm a junior in Java, facing a memory leaking problem. I always thought java won't have a memory problem but I got one. Below is my testing code.

I create a large ArrayList in getList() method, and keep calling this method in the main thread. I thought the list:

ArrayList<String> list = t.getList();

in loop is a local var, and should be released in the next round because a new object would be created. But jvm didn't do that. This thread keep eating all the memory. Would anyone tell me the reason? Thanks!

Testing code:

package test;

import java.util.ArrayList;

public class test {


public static void main(String[] args){

    test t = new test();
    for(int k = 0; k < 10000; k ++){
        ArrayList<String> list = t.getList();

        System.out.println(k);
    }

}

public ArrayList<String> getList(){
    ArrayList<String> list = new ArrayList<String>();

    for(int i = 0; i < 100000; i ++){
        String a = "";
        for(int j = 0; j < 100; j++){
            a += "aaaa";
        }
        list.add(a);
    }

    return list;
}
}

Modified Problem: in the modified problem, the thread also keep eating memory. But I thought it should release the memory during each round, but it didn't. So it occupied more and more memory.

public static void main(String[] args){

    test t = new test();
    for(int k = 0; k < 10000; k ++){
        String str1 = "string1";
        String str2 = "string2";
        double value = t.calculate(str1, str2);

        System.out.println(k + " " + value);
    }

}

public double calculate(String str1, String str2){
    double value = 0.0;

    ArrayList<String> list1 = getList(str1);
    ArrayList<String> list2 = getList(str2);

    value = cal(list1, list2); //will get the value based on lists

    return value;

}


public ArrayList<String> getList(String str1){
    ArrayList<String> list = new ArrayList<String>();

    //will generate  a long list

    return list;
}

Answer 1

I'm guessing that it's because of the iterated string concatenation:

a += "aaaa";

Not because the List isn't being released.

At the end of it all, a has 40,000,000 characters in it.

Answer 2

One call to getList will allocate space for 2000000000 characters (this includes the intermediate results for the string concatenations). Thats 4GB, not counting overhead. All this is done in a loop 10000 times giving total allocations of 40TB.

GC is doing a good job if this program executes for a while.

Answer 3

Common misconception is that the GC releases memory as soon at it not needed. The cost of running the GC is high, and so it is delayed as long as possible (Unless you use Azul's Zing in which case it is always concurrent)

This means it is completely normal to see memory consumption increase as you create more and more garbage and the ONLY way to know if you have a memory leak is to look at consumption AFTER a Full GC. Until a Full GC is run you can be sure that some objects which might have been cleaned up have not been.

But I thought it should release the memory during each round, but it didn't.

There is no way for the GC to know this is a good time to run. If you want to confirm this would be a good time, you can do a

System.gc();

between runs. The memory usage after call this should be pretty stable. The reason this call is not used more often is that usually it makes things worse, unless you know for example, that at that point you have dumped almost all the memory the application has used.

facing a memory leaking problem. I always thought java won't have a memory problem but I got one.

You can have a memory leak in a Java program, but I don't see any evidence you have one.

If you want to see a memory leak save all the ArrayList in an ArrayList in main. This should lead to an OutOfMemoryError and you have a leak.