Netty堆缓存问题

Myth丶恋晨 2023-07-25 12:49 18阅读 0赞

### Netty堆缓存问题 ###

*  1、问题描述
 *  2、问题分析
 *  3、问题解决
 *  4、总结

# 1、问题描述 #

今天学习`Netty`堆缓存和直接缓存遇到一个问题，明明使用的是堆缓存，这么读取不到数据呢？打印日志一看`heapBuf.hasArray()`直接返回`false`。来下面我们来看看源码，到底是怎么回事。

# 2、问题分析 #

首先写一个测试方法，直接向`ByteBuf`写入`中国万岁!`，然后如果是堆内存直接打印即可。源码如下：

@Test
        public void testHeapBuffer2() { 
            //取得堆内存 (但是默认是 directByDefault=true)
            ByteBuf heapBuf = ByteBufAllocator.DEFAULT.buffer();
            heapBuf.writeBytes("中国万岁！".getBytes(UTF_8));
            if (heapBuf.hasArray()) { 
                //取得内部数组
                byte[] array = heapBuf.array();
                int offset = heapBuf.arrayOffset() + heapBuf.readerIndex();
                int length = heapBuf.readableBytes();
                Logger.info("---------chen------------>" + new String(array, offset, length, UTF_8));
            }
            System.out.println("heapBuf.hasArray(): " + heapBuf.hasArray());
            heapBuf.release();
    
        }

控制台输出如下：

09:55:28.154 [main] DEBUG io.netty.util.internal.logging.InternalLoggerFactory - Using SLF4J as the default logging framework
    09:55:28.213 [main] DEBUG io.netty.util.internal.PlatformDependent0 - java.nio.Buffer.address: available
    09:55:28.216 [main] DEBUG io.netty.util.internal.PlatformDependent0 - sun.misc.Unsafe.theUnsafe: available
    09:55:28.217 [main] DEBUG io.netty.util.internal.PlatformDependent0 - sun.misc.Unsafe.copyMemory: available
    09:55:28.217 [main] DEBUG io.netty.util.internal.PlatformDependent0 - java.nio.Bits.unaligned: true
    09:55:28.218 [main] DEBUG io.netty.util.internal.PlatformDependent - Platform: Windows
    09:55:28.218 [main] DEBUG io.netty.util.internal.PlatformDependent - Java version: 8
    09:55:28.218 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noUnsafe: false
    09:55:28.218 [main] DEBUG io.netty.util.internal.PlatformDependent - sun.misc.Unsafe: available
    09:55:28.219 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noJavassist: false
    09:55:28.221 [main] DEBUG io.netty.util.internal.PlatformDependent - Javassist: unavailable
    09:55:28.221 [main] DEBUG io.netty.util.internal.PlatformDependent - You don't have Javassist in your class path or you don't have enough permission to load dynamically generated classes.  Please check the configuration for better performance.
    09:55:28.222 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.tmpdir: C:\Users\ADMINI~1\AppData\Local\Temp (java.io.tmpdir)
    09:55:28.222 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.bitMode: 64 (sun.arch.data.model)
    09:55:28.222 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noPreferDirect: false
    09:55:28.225 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.allocator.type: unpooled
    09:55:28.226 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.threadLocalDirectBufferSize: 65536
    09:55:28.228 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.maxThreadLocalCharBufferSize: 16384
    09:55:28.241 [main] DEBUG io.netty.buffer.AbstractByteBuf - -Dio.netty.buffer.bytebuf.checkAccessible: true
    09:55:28.245 [main] DEBUG io.netty.util.ResourceLeakDetector - -Dio.netty.leakDetection.level: simple
    09:55:28.245 [main] DEBUG io.netty.util.ResourceLeakDetector - -Dio.netty.leakDetection.maxRecords: 4
    heapBuf.hasArray(): false
    09:55:28.253 [main] DEBUG io.netty.util.internal.Cleaner0 - java.nio.ByteBuffer.cleaner(): available

是的，我们看到了`heapBuf.hasArray(): false`。`ByteBufAllocator.DEFAULT.buffer();`不是使用的是堆缓存吗？怎么会是`false`呢？纸上得来终觉浅，绝知此事要躬行。直接看源码。他的实现方法如下：

@Override
        public ByteBuf buffer() { 
            if (directByDefault) { 
                return directBuffer();
            }
            return heapBuffer();
        }

嗯，到底是堆内存还是直接内存还和`directByDefault`变量有关。我们在查找这个变量是在哪里赋值的。没错就是他的构造函数。

/** * Create new instance * * @param preferDirect {@code true} if {@link #buffer(int)} should try to allocate a direct buffer rather than * a heap buffer */
        protected AbstractByteBufAllocator(boolean preferDirect) { 
            directByDefault = preferDirect && PlatformDependent.hasUnsafe();
            emptyBuf = new EmptyByteBuf(this);
        }

是的，我们还没有找到答案，因为他还是和`preferDirect`变量相关。继续往下找。我们发现`UnpooledByteBufAllocator`的构造函数设置了这个值。

/** * Create a new instance * * @param preferDirect {@code true} if {@link #buffer(int)} should try to allocate a direct buffer rather than * a heap buffer */
        public UnpooledByteBufAllocator(boolean preferDirect) { 
            super(preferDirect);
        }

继续寻找`preferDirect`的值。

/** * Default instance */
        public static final UnpooledByteBufAllocator DEFAULT =
                new UnpooledByteBufAllocator(PlatformDependent.directBufferPreferred());

真实值是`PlatformDependent.directBufferPreferred()`，再次追踪进去。

/** * Returns {@code true} if the platform has reliable low-level direct buffer access API and a user has not specified * {@code -Dio.netty.noPreferDirect} option. */
        public static boolean directBufferPreferred() { 
            return DIRECT_BUFFER_PREFERRED;
        }

这个`DIRECT_BUFFER_PREFERRED`是何方神圣呢？继续追踪。

private static final boolean DIRECT_BUFFER_PREFERRED =
                HAS_UNSAFE && !SystemPropertyUtil.getBoolean("io.netty.noPreferDirect", false);

终于找到了作妖的源头了。是的就是他。根据定义我们知道`DIRECT_BUFFER_PREFERRED`默认是`True`，也就是默认是直接内存。`OMG`

# 3、问题解决 #

既然知道你是直接内存了，那我们直接改属性不就完了。通过`System.setProperty("io.netty.noPreferDirect", "true");`直接设置使用堆缓存。修改代码如下：

//堆缓冲区
        @Test
        public void testHeapBuffer() { 
            System.setProperty("io.netty.noPreferDirect", "true");
    
            //取得堆内存 (但是默认是 directByDefault=true)
            ByteBuf heapBuf = ByteBufAllocator.DEFAULT.buffer();
            heapBuf.writeBytes("中国万岁！".getBytes(UTF_8));
            if (heapBuf.hasArray()) { 
                //取得内部数组
                byte[] array = heapBuf.array();
                int offset = heapBuf.arrayOffset() + heapBuf.readerIndex();
                int length = heapBuf.readableBytes();
                Logger.info("---------chen------------>" + new String(array, offset, length, UTF_8));
            }
            heapBuf.release();
    
        }

直接运行，查看控制台

10:11:36.587 [main] DEBUG io.netty.util.internal.logging.InternalLoggerFactory - Using SLF4J as the default logging framework
    10:11:36.636 [main] DEBUG io.netty.util.internal.PlatformDependent0 - java.nio.Buffer.address: available
    10:11:36.638 [main] DEBUG io.netty.util.internal.PlatformDependent0 - sun.misc.Unsafe.theUnsafe: available
    10:11:36.639 [main] DEBUG io.netty.util.internal.PlatformDependent0 - sun.misc.Unsafe.copyMemory: available
    10:11:36.639 [main] DEBUG io.netty.util.internal.PlatformDependent0 - java.nio.Bits.unaligned: true
    10:11:36.640 [main] DEBUG io.netty.util.internal.PlatformDependent - Platform: Windows
    10:11:36.640 [main] DEBUG io.netty.util.internal.PlatformDependent - Java version: 8
    10:11:36.640 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noUnsafe: false
    10:11:36.640 [main] DEBUG io.netty.util.internal.PlatformDependent - sun.misc.Unsafe: available
    10:11:36.641 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noJavassist: false
    10:11:36.642 [main] DEBUG io.netty.util.internal.PlatformDependent - Javassist: unavailable
    10:11:36.642 [main] DEBUG io.netty.util.internal.PlatformDependent - You don't have Javassist in your class path or you don't have enough permission to load dynamically generated classes.  Please check the configuration for better performance.
    10:11:36.643 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.tmpdir: C:\Users\ADMINI~1\AppData\Local\Temp (java.io.tmpdir)
    10:11:36.643 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.bitMode: 64 (sun.arch.data.model)
    10:11:36.643 [main] DEBUG io.netty.util.internal.PlatformDependent - -Dio.netty.noPreferDirect: true
    10:11:36.646 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.allocator.type: unpooled
    10:11:36.647 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.threadLocalDirectBufferSize: 65536
    10:11:36.649 [main] DEBUG io.netty.buffer.ByteBufUtil - -Dio.netty.maxThreadLocalCharBufferSize: 16384
    10:11:36.659 [main] DEBUG io.netty.buffer.AbstractByteBuf - -Dio.netty.buffer.bytebuf.checkAccessible: true
    10:11:36.664 [main] DEBUG io.netty.util.ResourceLeakDetector - -Dio.netty.leakDetection.level: simple
    10:11:36.664 [main] DEBUG io.netty.util.ResourceLeakDetector - -Dio.netty.leakDetection.maxRecords: 4
    [main|BufferTypeTest.testHeapBuffer] |>  ---------chen------------>中国万岁！

`[main|BufferTypeTest.testHeapBuffer] |> ---------chenwei------------>中国万岁！`完美输出结果。问题解决。

# 4、总结 #

书上的代码直接运行绝大部分是对的，但是总有一些软件的更新使得作者无能为力。之前的API是对的，但是之后就废弃了或修改了是常有的事。所以我们需要跟踪源代码。这只是一个小小的问题，如果没有前辈的无私奉献，很难想象我们自己一天能学到多少内容。感谢各位前辈的辛勤付出，让我们少走了很多的弯路！

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