ByteBuffer的allocate()方法详解
ByteBuffer的allocate()方法详解
allocate()方法用于分配缓冲区。但是如果是聚集写入,与分散读取,就需要注意这个大小设置,
String property = System.getProperty("user.dir");
FileOutputStream FileOutputStream = new FileOutputStream(property + "/lib/aab.text");
FileChannel channel = FileOutputStream.getChannel();
ByteBuffer allocate1 = ByteBuffer.allocate(90);
ByteBuffer allocate2 = ByteBuffer.allocate(400);
allocate1.asCharBuffer().put("400");
allocate2.asCharBuffer().put("The world is so big, I want to see it.");
channel.write(new ByteBuffer[]{allocate1,allocate2});
channel.close();
这里有两个buffer,分别分配缓冲区90字节,400字节,下面读取输出:
String property = System.getProperty("user.dir");
FileInputStream FileInputStream = new FileInputStream(property + "/lib/aab.text");
FileChannel channel = FileInputStream.getChannel();
ByteBuffer allocate1 = ByteBuffer.allocate(90);
ByteBuffer allocate2 = ByteBuffer.allocate(400);
channel.read(new ByteBuffer[]{allocate1,allocate2});
allocate1.rewind();
allocate2.rewind();
String i = allocate1.asCharBuffer().toString();
String string = allocate2.asCharBuffer().toString();
System.out.println(1+" : "+i);
System.out.println(2+" : "+string);
channel.close();
结果
1 : 400
2 : The world is so big, I want to see it.
我们修改一下读取代码中缓冲区大小
String property = System.getProperty("user.dir");
FileInputStream FileInputStream = new FileInputStream(property + "/lib/aab.text");
FileChannel channel = FileInputStream.getChannel();
ByteBuffer allocate1 = ByteBuffer.allocate(8);//改为8
ByteBuffer allocate2 = ByteBuffer.allocate(80);//改为80
channel.read(new ByteBuffer[]{allocate1,allocate2});
allocate1.rewind();
allocate2.rewind();
String i = allocate1.asCharBuffer().toString();
String string = allocate2.asCharBuffer().toString();
System.out.println(1+" : "+i);
System.out.println(2+" : "+string);
channel.close();
结果
1 : 400
2 :
这里就没有读取到,所以哪怕持久化到硬盘,两个缓冲区任然是两个缓冲区,第一个缓冲区的90字节没有读取完,是不会去第二个缓冲区,
| 第一个缓冲区(90字节) | 第二个缓冲区(400字节) |
|---|---|
| 400 | The world is so big, I want to see it. |
再来修改一下代码
String property = System.getProperty("user.dir");
FileInputStream FileInputStream = new FileInputStream(property + "/lib/aab.text");
FileChannel channel = FileInputStream.getChannel();
ByteBuffer allocate1 = ByteBuffer.allocate(170);//改为170
ByteBuffer allocate2 = ByteBuffer.allocate(400);
channel.read(new ByteBuffer[]{allocate1,allocate2});
allocate1.rewind();
allocate2.rewind();
String i = allocate1.asCharBuffer().toString();
String string = allocate2.asCharBuffer().toString();
System.out.println(1+" : "+i);
System.out.println(2+" : "+string);
channel.close();
结果
1 : 400The world is so big, I want to see it.
2 :
这里说明每个缓冲区的内容在最开始位置,"400"只占用了6个字节,"The world is so big, I want to see it."占用了76个字节.当第一缓冲区的90字节读取完,进入第二缓冲区,170-90=80>76,完全可以读到第二缓冲区的文本内容