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java入门要注意什么

学习java就像是一个种花的过程，不断地为其施肥浇水，它才会茁壮成长。 而我们学习java，就要不断的充实自己、提升自己，才能获得更多机会。很多开始学习java编程的小白，经常就会被概念、定义什么的搞糊涂。当分类 、对象、接口、构造函数等等各种专业名词出现的时候，你一定是脑子里好像一片空白，根本就搞不懂这些字眼的意思和关系，而且，这种情况下，很 容易导致你丧失自信心，开始逃避、拒绝，这些小白经常遇到的情况在我刚接触java的时候也遇见了，但是好在我足够幸运，遇见了诚筑说。我现在已 经是公司的项目经理了，今天，我为大家来总结了一些经验和建议，希望能够帮助到大家。

一点：熟练基本的j2seAPI

除去java语言本身的语法之外呢，要懂得并且熟练j2seAPI的API也是非常有 必要的，在这里，就建议大家首先去掌握字符串的处理、异常的处理、容器、输入输出、线程等，这些相对来说较为重要的。还有就是API的内容是非 常庞大的，关于API，一定要懂得查询API的文件说明，在了解了其作用用途或者目的才能够进行相对于的程序。

二点：稳固java的语法基础

学习java一定要学会使用java的程序语言，用来编写程序，但是学习程序语 言就要熟悉语法是怎么使用的。程序语言其实也是一种语言，不过跟人类的语言不同，这种语言是要和计算机沟通交流，那怎么做才能熟悉这种语言呢 ，我给出的建议是多看别人写的程序，了解人家是怎么用java来解决问题的。然后再找类似的程序去练习了，这样就能够从实际操作中检验自己是否真 的知道该怎么去解决问题了。

三点：加入贴吧论坛多参与讨论

根据我当时的经验，在大家学习的过程中，如果有人可以参与话题，共同讨 论的话，会加快你学习的速度。所以大家可以和我一样，找一个技术讨论的地方，贴吧啊，论坛啊都可以，在这里进行讨论，毕竟大家有着共同的目标 和理想，有着共同的话题可聊，这样的话，又大大节省了学习的时间。

学完基本的java语法呢，现在就该用java来进行实际的编程了，假如你需要 编写窗口程序，那就学Swing窗口设计；假如你要编写数据库什么的，那就学JDBC等等。

Java开发体系结构介绍 ：

Java开发体系结构介绍 ：

1、类加载器：为程序的执行加载所需要的全部类。类加载器将本地文件系 统的类名空间与来自远程网络源的类名空间相分离，本地类总是首先被加载，以增加安全性。当全部类被加载后，可执行文件的存储器格式被确定。这 时，特定的存储器地址被分配给符号引用并创建检索表格。由于存储器格式在运行时出现，因而Java解释器增加了保护以防止对限制代码区的非法进入 。

2、字节代码校验器：基于代码的规范包括语法语义的检查以及如上所述的 安全性检查。

3、Java运行时解释器：它是JVM的核心内容，实现把抽象的字节码指令映射 到本地系统平台下的库引用或指令。

4、API类库：实现标准Java平台API的一系列可执行代码。

5、硬件本地平台接口：提供对底层系统平台资源库调用的接口。

BLE CoC in AndroidWhy do we need BLE CoC?What is BLE CoC?Does Android support BLE CoC?

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Why do we need BLE CoC?

The target markets for Bluetooth Low Energy (BLE) include the smart home, health, sport and fitness areas. All early BLE PRofiles, like BLP, CSCP and HOGP, are based on GATT, a general specification for sending and receiving short pieces of data known as attributes over a low energy link. The size of data may only be several or a dozen bytes. The data could be temperature, speed, heart-rate and so on. All of them are short pieces of data. The fixed channel for GATT is suitable for the short pieces of data. The default protocol payload for the channel is limited to the default MTU of L2CAP for BLE, 23 bytes which are enough for the short pieces of data.

However, the new requirements for transferring the large amounts of data have arisen. For example, syncing GPX file and updating firmware, the size of data may be dozens of kilobytes and even more than 100 KB. Let’s say the size is 65535 Bytes (65 KB), we need 2850 L2CAP packets to transfer the data (ceil(65535 / 23) = 2850). There are two drawbacks: lowering throughput rate and blocking protocols multiplex.

2850 L2CAP basic headers and ATT parameters for write command have to be transferred, that wastes the bandwidth. It needs more CPU cycles to split the large service data into many short pieces of L2CAP payload.The higher layers are aware of only their own needs and do not worry about multiplexing other protocols – this functionality is assumed in L2CAP. ATT will reserve next 2850 packets on radio. During this transfer, no other interactions (example SMP) can occur.

L2CAP Connection Oriented Channel in addition to MTU provides the concept of MPS which determines the size of segments that should be used to send a large chunk of data. The value of MPS may be 65533, so we can transfer a large amounts of data in a LE-frame. L2CAP Connection Oriented Channel has an explicit connection procedure to the Protocol Service Multiplexer (PSM) identifying the protocol or service that you need to connect to. Currently, Internet Protocol Support Profile (IPSP) and Object Transfer Profile (OTP) are the two users of BLE CoC.

What is BLE CoC?

Connection-oriented channel (CoC) is defined in L2CAP. CoC may be created for transport of unicast (point-to-point) data between two Bluetooth devices. QoS parameters or flow and error control modes may be applied in CoC. L2CAP channels may be connection-oriented or connectionless. Fixed channels other than the L2CAP connectionless channel (CID 0x0002) and the two L2CAP signaling channels (CIDs 0x0001 for classic and 0x0005 for LE) are considered connection-oriented. All channels with dynamically assigned CIDs are connection-oriented.

There are three modes for connection-oriented channel

Connection-oriented channel in basic L2CAP modeConnection-oriented channel in retransmission/flow control modeConnection-oriented channel in LE credit based flow control mode

BLE CoC is only in LE credit based flow control mode in which the L2CAP PDU on a connection-oriented channel is LE-frame.

*L2CAP SDU Length field is only contained in the first LE-frame of the SDU to specify the total number of octets in the SDU. All subsequent LE-frames that are parts of the same SDU shall not contain the L2CAP SDU Length field.

MTU = maximum SDU size

MPS = maximum PDU payload size

The local device sends LE Credit Based Connection Request packet to create a connection. The remote device sendsLE Credit Based Connection Response packet when receives the request packet. The local and remote devices inform/negotiate the parameters to create and configure an L2CAP connection-oriented channel in theConfiguration Process. These parameters areLE_PSM, Source/DestinationCID,MTU,MPS and Initial Credits.

The initial credit value indicates the number of LE-frames that the peer device can send to the local device. The initial credit value shall be in the range of 0 to 65535. LE-frames shall only be sent on LE CoC if the device has a credit count greater than zero for that channel. For each LE-frame sent the device decreases the credit count for that channel by one. The peer device may send aLE Flow Control Credit packet to increase the credit count for that channel by the value of the Credits field in this packet. One credit represents the permission to send one LE-frame over the established channel. The attempt to send more LE-frames than the permitted results in the receiving device closing the channel.

A Protocol Service Multiplexer (PSM) must be associated with any credit-based connection. LE_PSM is a two-byte value identifying the protocol that uses the credit-based connection. LE_PSM values are separated into two ranges which are defined in the table below.

*0x0023 is reserved by Internet Protocol Support Profile (IPSP), and 0x0025 is reserved by Object Transfer Profile (OTP).

Let’s assume that device A sends 95 bytes SDU to device B.

The first LE-frame Payload starts with 2 bytes of SDU Length ‘95’ then follow the first MPS-2 bytes of SDU. The second LE-frame contains MPS bytes of SDU. The last LE-frame contains less than MPS bytes.

Does Android support BLE CoC?

LE CoC is a feature of Core spec v4.1. Theimplementation for LE CoC has been merged in AOSP, but the APIs are hidden. So app cannot use the APIs which are not exposed in Android 7.

BluetoothAdapter::listenUsingL2capOn() is used to create L2CAP server socket, including LE CoC. A specified value of parameter ‘port’ is passed to Fluoride Bluetooth stack to create LE CoC. ‘L2CAP_MASK_LE_COC_CHANNEL’ is used to check if the value of ‘channel’ corresponding to ‘port’ is for LE CoC in Fluoride BT stack. The snippet of setting ‘is_le_coc’ flag is in aosp/system/bt/btif/src/btif_sock_l2cap.c.

How to create a BLE CoC server in Android?

BluetoothAdapter::listenUsingL2capOn() is not exposed to app developer in Android 7, but there are still two method to create LE CoC server socket.

Method 1: Remove ‘@hide’, and rebuild AOSP to create the customized image and SDK. Then directly call BluetoothAdapter::listenUsingL2capOn() in your app.

Method 2: Call BluetoothAdapter::listenUsingL2capOn() using java reflection on AOSP image. The example snippet is below, and you can access my github to get source code.

If the LE CoC server socket is created successfully, you can see the logs below in logcat.

Note: The two method are only for experimental purpose. For example, you can test LE CoC between your wearable device and Android phone.

Reference: 1. Bluetooth core spec v4.1 2. <Bluetooth LE Credit-Based Flow Control for L2CAP Connection-Oriented Channels> https://community.nxp.com/thread/366041 3. <L2CAP oriented connection> https://devzone.nordicsemi.com/question/60552/l2cap-oriented-connection/

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