java中的NIO和IO到底是什麼區別？20個問題告訴你答案

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通道的write和read方法都支持傳入1個buffer數組，會按照順序做讀寫操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/a3/a3b6f63f2818332b26c6899a8529796c.jpeg","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/36/360e5795403b9963dce064e08723fd95.jpeg","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"​Buffer的種類：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c1/c1b64b3984a5cbd7c613b9362986a0fd.jpeg","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"​Buffer的另外3個方法：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"warp:","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"根據一個byte[]來生成一個固定的ByteBuffer時，使用ByteBuffer.wrap()非法的合適。他會直接基於byte[]數組生成一個新的buffer，值也保持一致。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"slice：","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"得到切片後的數組。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"duplicate：","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"調用duplicate方法返回的Buffer對象就是複製了一份原始緩衝區，複製了position、limit、capacity這些屬性","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意！","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 以上warp\\slice\\duplicte生成的緩衝區get和put所操作的數組還是與原始緩衝區一樣的","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic","attrs":{}},{"type":"strong","attrs":{}}],"text":" 所以對複製後的緩衝區進行修改也會修改原始的緩衝區，反之亦然","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 因此duplicte、slice一般是用於操作一下poistion\\limit等處理，但是原內容不會去變他，否則就會引起 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在NIO中Selector的好處是什麼？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以用更少的線程來管理各個通道。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"減少線程上下文切換的資源開銷。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Q： Selector支持註冊哪種類型的通道？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：支持非阻塞的通道。通道要在註冊前調用channel.configureBlocking(false) 設置爲非阻塞。例如FileChannel就沒辦法註冊，他註定是阻塞的。而socketChannel就可以支持非阻塞。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Q： Selector註冊時，支持監聽哪幾種事件，對應的常量是什麼？（啊最不喜歡記憶這種東西了…）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：共有4種可監聽事件","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Connect 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如果希望對該通道監聽多種事件，可以用\"|\"位或操作符把常量連接起來。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"int interestingSet = Selectionkey.OP_READ | Selectionkey.OP_WRITE;\nSelectionkey key = channel.register(selector,interestingSet)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"SelectionKey鍵表示了一個特定的通道對象和一個特定的選擇器對象之間的註冊關係","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Q： 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set)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所有與選擇器關聯的通道所生成的鍵的集合稱爲已經註冊的鍵的集合。並不是所有註冊過的鍵都仍然有效。這個集合通過keys()方法返回，並且可能是空的。這個已註冊的鍵的集合不是可以直接修改的；試圖這麼做的話將引發java.lang.UnsupportedOperationException。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2.   ","attrs":{}},{"type":"text","marks":[{"type":"italic","attrs":{}},{"type":"strong","attrs":{}}],"text":"已選擇的鍵的集合(Selected key 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set)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"已註冊的鍵的集合的子集，這個集合包含了cancel()方法被調用過的鍵(這個鍵已經被無效化)，但它們還沒有被註銷。這個集合是選擇器對象的私有成員，因而無法直接訪問。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"註冊之後， 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是否有事件就緒了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"5.   如果有事件就緒，從key中獲取對應對應管道。做對應處理","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"類似如下，一般都會啓1個線程來run這個selector監聽的處理：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"while(true) {\n int readyNum = selector.select();\n if (readyNum == 0) {\n continue;\n }\n \n Set selectedKeys = selector.selectedKeys();\n Iterator it = selectedKeys.iterator();\n\t\t\n while(it.hasNext()) {\n SelectionKey key = it.next();\n if(key.isAcceptable()) {\n // 接受連接\n } else if (key.isReadable()) {\n // 通道可讀\n } else if (key.isWritable()) {\n // 通道可寫\n }\n \n it.remove();\n }\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"​Q：select()方法其實是阻塞方法，即調用時會進入等待，直到把所有通道都輪詢完畢。如果希望提前結束select()，有哪些方法？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：有2個辦法：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"wakeup()， 調用後，select（）方法立刻返回。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"close()， 直接關閉selector。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"PS: 之前說NIO是非阻塞IO，但爲什麼上面卻說select()方法是阻塞的？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其實NIO的非阻塞，指的是IO不阻塞，即我們不會卡在read()處，我們會用selector去查詢就緒狀態，如果狀態ok就。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而查詢操作是需要時間，因此select()必須要把所有通道都檢查一遍才能告訴結果，因此select這個查詢操作是阻塞的。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"其他","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Q： 多線程讀寫同一文件時，如何加鎖保證線程安全？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：使用FileChannel的加鎖功能。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"RandomAccessFile randFile = new RandomAccessFile(target, \"rw\");\nFileChannel channel = randFile.getChannel();\n// pos和siz決定加鎖區域， shared指定是否是共享鎖\nFileLock fileLock = channel.lock(pos , size , shared);\nif (fileLock!=null) {\n\tdo();\n // 這裏簡化了，實際上應該用try-catch\n\tfileLock.release();\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"​Q： 如果需要讀1個特大文件，可以使用什麼緩衝區？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A：使用MappedByteBuffer。這個緩衝區可以把大文件理解成1個byte數組來訪問（但實際上並沒有加載這麼大的byte數組，實際內容放在內存+虛存中）。主要通過FileChannel.map（模式，起始位置，區域）來生成1個MappedByteBuffer。然後可以用put和get去處理對應位置的byte。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"int length = 0x8FFFFFF;//一個byte佔1B，所以共向文件中存128M的數據\ntry (FileChannel channel = FileChannel.open(Paths.get(\"src/c.txt\"),\n\t\tStandardOpenOption.READ, StandardOpenOption.WRITE);) {\n\tMappedByteBuffer mapBuffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, length);\n\tfor(int i=0;i