java直接内存与堆内存的比较

╰半橙微兮° 2024-02-17 16:39 56阅读 0赞

对于nio这个库可以直接操作内存的使用，可以做个对比，看哪种情况下适合使用。

import java.nio.ByteBuffer;
    
    public class AccessDirectBuffer {
        //直接分配内存
        public static void directAccess() {
            long startTime = System.currentTimeMillis();
            ByteBuffer b = ByteBuffer.allocateDirect(500);
            for (int i = 0; i < 100000; i++) {
                for (int j = 0; j < 99; j++)
                    b.putInt(j);
                b.flip();
                for (int j = 0; j < 99; j++)
                    b.getInt();
                b.clear();
            }
            long endTime = System.currentTimeMillis();
            System.out.println("directAccess:" + (endTime - startTime));
        }
    
        //分配堆内存
        public static void bufferAccess() {
            long startTime = System.currentTimeMillis();
            ByteBuffer b = ByteBuffer.allocate(500);
            for (int i = 0; i < 100000; i++) {
                for (int j = 0; j < 99; j++)
                    b.putInt(j);
                b.flip();
                for (int j = 0; j < 99; j++)
                    b.getInt();
                b.clear();
            }
            long endTime = System.currentTimeMillis();
            System.out.println("bufferAccess" + (endTime - startTime));
        }
    
        public static void main(String args[]) {
    
            bufferAccess();
            directAccess();
            bufferAccess();
            directAccess();
    
        }
    }

输出结果：

bufferAccess57  
directAccess:28  
bufferAccess55  
directAccess:10

可以看到直接内存读写有较大优势，但是看看下面的情况：

import java.nio.ByteBuffer;
    
    public class AllocDirectBuffer {
        public static void directAllocate() {
            long startTime = System.currentTimeMillis();
            for (int i = 0; i < 200000; i++) {
                ByteBuffer.allocateDirect(1000);
            }
            long endTime = System.currentTimeMillis();
            System.out.println("directAllocate:" + (endTime - startTime));
        }
    
        public static void bufferAllocate() {
            long startTime = System.currentTimeMillis();
            for (int i = 0; i < 200000; i++) {
                ByteBuffer.allocate(1000);
            }
            long endTime = System.currentTimeMillis();
            System.out.println("bufferAllocate:" + (endTime - startTime));
        }
    
        public static void main(String[] args) {
            bufferAllocate();
            directAllocate();
            bufferAllocate();
            directAllocate();
        }
    }

输出结果：

bufferAllocate:38  
directAllocate:282  
bufferAllocate:71  
directAllocate:403

可以看到在内存空间申请上，直接内存毫无优势。

如果内存空间本身需要频繁申请，则不适合使用直接内存。