Android系统匿名共享内存Ashmem（Anonymous Shared Memory）简要介绍和学习计划 (四)

 这个Server继承了Android系统应用程序框架层提供的Service类，当它被启动时，运行在一个独立的进程中。当这个Server被启动时，它的onCreate函数就会被调用，然后它就通过ServiceManager的addService接口来添加MemoryService了：

 

 

memoryService = new MemoryService();

try {
    ServiceManager.addService("AnonymousSharedMemory", memoryService);
    Log.i(LOG_TAG, "Succeed to add memory service.");
} catch (RuntimeException ex) {
    Log.i(LOG_TAG, "Failed to add Memory Service.");
    ex.printStackTrace();
}

memoryService = new MemoryService();

try {
    ServiceManager.addService("AnonymousSharedMemory", memoryService);
    Log.i(LOG_TAG, "Succeed to add memory service.");
} catch (RuntimeException ex) {
    Log.i(LOG_TAG, "Failed to add Memory Service.");
    ex.printStackTrace();
}

    memoryService = new MemoryService();

    try {
        ServiceManager.addService("AnonymousSharedMemory", memoryService);
        Log.i(LOG_TAG, "Succeed to add memory service.");
    } catch (RuntimeException ex) {
        Log.i(LOG_TAG, "Failed to add Memory Service.");
        ex.printStackTrace();
    }

       这样，当这个Server成功启动了，Client就可以通过ServiceManager的getService接口来获取这个MemoryService了。

 

       接着，我们就来看Client端的实现。Client端是一个Activity，实现在src/shy/luo/ashmem/Client.java文件中：

 

package shy.luo.ashmem;

import java.io.FileDescriptor;
import java.io.IOException;

import shy.luo.ashmem.R;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.MemoryFile;
import android.os.ParcelFileDescriptor;
import android.os.ServiceManager;
import android.os.RemoteException;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Client extends Activity implements OnClickListener {
    private final static String LOG_TAG = "shy.luo.ashmem.Client";

    IMemoryService memoryService = null;
    MemoryFile memoryFile = null;

    private EditText valueText = null;
    private Button readButton = null;
    private Button writeButton = null;
    private Button clearButton = null;

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main);

        IMemoryService ms = getMemoryService();
        if(ms == null) {
                startService(new Intent("shy.luo.ashmem.server"));
        } else {
            Log.i(LOG_TAG, "Memory Service has started.");
        }

            valueText = (EditText)findViewById(R.id.edit_value);
            readButton = (Button)findViewById(R.id.button_read);
            writeButton = (Button)findViewById(R.id.button_write);
            clearButton = (Button)findViewById(R.id.button_clear);

        readButton.setOnClickListener(this);
            writeButton.setOnClickListener(this);
            clearButton.setOnClickListener(this);

            Log.i(LOG_TAG, "Client Activity Created.");
        }

        @Override
        public void onResume() {
        super.onResume();

        Log.i(LOG_TAG, "Client Activity Resumed.");
        }

        @Override
        public void onPause() {
        super.onPause();

        Log.i(LOG_TAG, "Client Activity Paused.");
        }

        @Override
        public void onClick(View v) {
            if(v.equals(readButton)) {
                int val = 0;

                MemoryFile mf = getMemoryFile();
                if(mf != null) {
                try {
                        byte[] buffer = new byte[4];
                        mf.readBytes(buffer, 0, 0, 4);

                        val = (buffer[0] << 24) | ((buffer[1] & 0xFF) << 16) | ((buffer[2] & 0xFF) << 8) | (buffer[3] & 0xFF);
                } catch(IOException ex) {
                    Log.i(LOG_TAG, "Failed to read bytes from memory file.");
                    ex.printStackTrace();
                }
                }

                String text = String.valueOf(val);
                valueText.setText(text);
            } else if(v.equals(writeButton)) {
                String text = valueText.getText().toString();
                int val = Integer.parseInt(text);

                IMemoryService ms = getMemoryService();
                if(ms != null) {
                try {
                        ms.setValue(val);
                } catch(RemoteException ex) {
                    Log.i(LOG_TAG, "Failed to set value to memory service.");
                    ex.printStackTrace();
                }
                }
            } else if(v.equals(clearButton)) {
                String text = "";
                valueText.setText(text);
            }
        }

        private IMemoryService getMemoryService() {
            if(memoryService != null) {
                return memoryService;
            }

            memoryService = IMemoryService.Stub.asInterface(
                            ServiceManager.getService("AnonymousSharedMemory"));

        Log.i(LOG_TAG, memoryService != null ? "Succeed to get memeory service." : "Failed to get memory service.");

            return memoryService;
        }

        private MemoryFile getMemoryFile() {
            if(memoryFile != null) {
                return memoryFile;
            }

            IMemoryService ms = getMemoryService();
            if(ms != null) {
            try {
                    ParcelFileDescriptor pfd = ms.getFileDescriptor();
                if(pfd == null) {
                    Log.i(LOG_TAG, "Failed to get memory file descriptor.");
                    return null;
                }

                try {
                    FileDescriptor fd = pfd.getFileDescriptor();
                    if(fd == null) {
                        Log.i(LOG_TAG, "Failed to get memeory file descriptor.");
                        return null;
                    }

                        memoryFile = new MemoryFile(fd, 4, "r");
                } catch(IOException ex) {
                    Log.i(LOG_TAG, "Failed to create memory file.");
                    ex.printStackTrace();
                }
                } catch(RemoteException ex) {
                Log.i(LOG_TAG, "Failed to get file descriptor from memory service.");
                ex.printStackTrace();
            }
        }

            return memoryFile;
        }
}

package shy.luo.ashmem;

import java.io.FileDescriptor;
import java.io.IOException;

import shy.luo.ashmem.R;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.MemoryFile;
import android.os.ParcelFileDescriptor;
import android.os.ServiceManager;
import android.os.RemoteException;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Client extends Activity implements OnClickListener {
    private final static String LOG_TAG = "shy.luo.ashmem.Client";

    IMemoryService memoryService = null;
    MemoryFile memoryFile = null;

    private EditText valueText = null;
    private Button readButton = null;
    private Button writeButton = null;
    private Button clearButton = null;

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.main);

        IMemoryService ms = getMemoryService();
        if(ms == null) {
                startService(new Intent("shy.luo.ashmem.server"));
        } else {
            Log.i(LOG_TAG, "Memory Service has started.");
        }

            valueText = (EditText)findViewById(R.id.edit_value);
            readButton = (Button)findViewById(R.id.button_read);
            writeButton = (Button)findViewById(R.id.button_write);
            clearButton = (Button)findViewById(R.id.button_clear);

        readButton.setOnClickListener(this);
            writeButton.setOnClickListener(this);
            clearButton.setOnClickListener(this);

            Log.i(LOG_TAG, "Client Activity Created.");
        }

        @Override
        public void onResume() {
        super.onResume();

        Log.i(LOG_TAG, "Client Activity Resumed.");
        }

        @Override
        public void onPause() {
        super.onPause();

        Log.i(LOG_TAG, "Client Activity Paused.");
        }

        @Override
        public void onClick(View v) {
            if(v.equals(readButton)) {
                int val = 0;

                MemoryFile mf = getMemoryFile();
                if(mf != null) {
                try {
                        byte[] buffer = new byte[4];
                        mf.readBytes(buffer, 0, 0, 4);

                        val = (buffer[0] << 24) | ((buffer[1] & 0xFF) << 16) | ((buffer[2] & 0xFF) << 8) | (buffer[3] & 0xFF);
                } catch(IOException ex) {
                    Log.i(LOG_TAG, "Failed to read bytes from memory file.");
                    ex.printStackTrace();
                }
                }

                String text = String.valueOf(val);
                valueText.setText(text);
            } else if(v.equals(writeButton)) {
                String text = valueText.getText().toString();
                int val = Integer.parseInt(text);

                IMemoryService ms = getMemoryService();
                if(ms != null) {
                try {
                        ms.setValue(val);
                } catch(RemoteException ex) {
                    Log.i(LOG_TAG, "Failed to set value to memory service.");
                    ex.printStackTrace();
                }
                }
            } else if(v.equals(clearButton)) {
                String text = "";
                valueText.setText(text);
            }
        }

        private IMemoryService getMemoryService() {
            if(memoryService != null) {
                return memoryService;
            }

            memoryService = IMemoryService.Stub.asInterface(
                            ServiceManager.getService("AnonymousSharedMemory"));

        Log.i(LOG_TAG, memoryService != null ? "Succeed to get memeory service." : "Failed to get memory service.");

            return memoryService;
        }

        private MemoryFile getMemoryFile() {
            if(memoryFile != null) {
                return memoryFile;
            }

            IMemoryService ms = getMemoryService();
            if(ms != null) {
            try {
                    ParcelFileDescriptor pfd = ms.getFileDescriptor();
                if(pfd == null) {
                    Log.i(LOG_TAG, "Failed to get memory file descriptor.");
                    return null;
                }

                try {
                    FileDescriptor fd = pfd.getFileDescriptor();
                    if(fd == null) {
                        Log.i(LOG_TAG, "Failed to get memeory file descriptor.");
                        return null;
                    }

                        memoryFile = new MemoryFile(fd, 4, "r");
                } catch(IOException ex) {
                    Log.i(LOG_TAG, "Failed to create memory file.");
                    ex.printStackTrace();
                }
                } catch(RemoteException ex) {
                Log.i(LOG_TAG, "Failed to get file descriptor from memory service.");
                ex.printStackTrace();
            }
        }

            return memoryFile;
        }
}

package shy.luo.ashmem;

import java.io.FileDescriptor;
import java.io.IOException;

import shy.luo.ashmem.R;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.MemoryFile;
import android.os.ParcelFileDescriptor;
import android.os.ServiceManager;
import android.os.RemoteException;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Client extends Activity implements OnClickListener {
	private final static String LOG_TAG = "shy.luo.ashmem.Client";
	
	IMemoryService memoryService = null;
	MemoryFile memoryFile = null;
	
	private EditText valueText = null;
	private Button readButton = null;
	private Button writeButton = null;
	private Button clearButton = null;
	
    	@Override
    	public void onCreate(Bundle savedInstanceState) {
        	super.onCreate(savedInstanceState);
        	setContentView(R.layout.main);

		IMemoryService ms = getMemoryService();
		if(ms == null) {        
        		startService(new Intent("shy.luo.ashmem.server"));
		} else {
			Log.i(LOG_TAG, "Memory Service has started.");
		}

        	valueText = (EditText)findViewById(R.id.edit_value);
        	readButton = (Button)findViewById(R.id.button_read);
        	writeButton = (Button)findViewById(R.id.button_write);
        	clearButton = (Button)findViewById(R.id.button_clear);

		readButton.setOnClickListener(this);
        	writeButton.setOnClickListener(this);
        	clearButton.setOnClickListener(this);
        
        	Log.i(LOG_TAG, "Client Activity Created.");
    	}

    	@Override
    	public void onResume() {
		super.onResume();

		Log.i(LOG_TAG, "Client Activity Resumed.");
    	}

    	@Override
    	public void onPause() {
		super.onPause();

		Log.i(LOG_TAG, "Client Activity Paused.");
    	}
    
    	@Override
    	public void onClick(View v) {
    		if(v.equals(readButton)) {
    			int val = 0;
    		
    			MemoryFile mf = getMemoryFile();
    			if(mf != null) {
				try {
    					byte[] buffer = new byte[4];
    					mf.readBytes(buffer, 0, 0, 4);
    			
    					val = (buffer[0] << 24) | ((buffer[1] & 0xFF) << 16) | ((buffer[2] & 0xFF) << 8) | (buffer[3] & 0xFF);
				} catch(IOException ex) {
					Log.i(LOG_TAG, "Failed to read bytes from memory file.");
					ex.printStackTrace();
				}
    			}	
    		
    			String text = String.valueOf(val);
    			valueText.setText(text);
    		} else if(v.equals(writeButton)) {
    			String text = valueText.getText().toString();
    			int val = Integer.parseInt(text);
    		
    			IMemoryService ms = getMemoryService();
    			if(ms != null) {
				try {
    					ms.setValue(val);
				} catch(RemoteException ex) {
					Log.i(LOG_TAG, "Failed to set value to memory service.");
					ex.printStackTrace();
				}
    			}
    		} else if(v.equals(clearButton)) {
    			String text = "";
    			valueText.setText(text);
    		}
    	}
    
    	private IMemoryService getMemoryService() {
    		if(memoryService != null) {
    			return memoryService;
    		}
    	
    		memoryService = IMemoryService.Stub.asInterface(
                			ServiceManager.getService("AnonymousSharedMemory"));

		Log.i(LOG_TAG, memoryService != null ? "Succeed to get memeory service." : "Failed to get memory service.");
    	
    		return memoryService;
    	}
    
    	private MemoryFile getMemoryFile() {
    		if(memoryFile != null) {
    			return memoryFile;
    		}
    		
    		IMemoryService ms = getMemoryService();
    		if(ms != null) {
			try {
    				ParcelFileDescriptor pfd = ms.getFileDescriptor();
				if(pfd == null) {
					Log.i(LOG_TAG, "Failed to get memory file descriptor.");
					return null;
				}

				try {
					FileDescriptor fd = pfd.getFileDescriptor();
					if(fd == null) {
						Log.i(LOG_TAG, "Failed to get memeory file descriptor.");
						return null;                      
					}	

    					memoryFile = new MemoryFile(fd, 4, "r");
				} catch(IOException ex) {
					Log.i(LOG_TAG, "Failed to create memory file.");
					ex.printStackTrace();
				}
    			} catch(RemoteException ex) {
				Log.i(LOG_TAG, "Failed to get file descriptor from memory service.");
				ex.printStackTrace();
			}
		}
    	
    		return memoryFile;
    	}
}

        Client端的界面主要包含了三个按钮Read、Write和Clear，以及一个用于显示内容的文本框。

 

        这个Activity在onCreate时，会通过startService接口来启动我们前面定义的Server进程。调用startService时，需要指定要启动的服务的名称，这里就是"shy.luo.ashmem.server"了，后面我们会在程序的描述文件AndroidManifest.xml看到前面的Server类是如何和名称"shy.luo.ashmem.server"关联起来的。关于调用startService函数来启动自定义服务的过程，可以参考Android系统在新进程中启动自定义服务过程（startService）的原理分析一文。

        内部函数getMemoryService用来获取IMemoryService。如果是第一次调用该函数，则会通过ServiceManager的getService接口来获得这个IMemoryService接口，然后保存在类成员变量memoryService中，以后再调用这个函数时，就可以直接返回memoryService了。

        内部函数getMemoryFile用来从MemoryService中获得匿名共享内存文件的描述符。同样，如果是第一次调用该函数，则会通过IMemoryService的getFileDescriptor接口来获得MemoryService中的匿名共享内存文件的描述符，然后用这个文件描述符来创建一个MemoryFile实例，并保存在类成员变量memoryFile中，以后再调用这个函数时，就可以直接返回memoryFile了。

        有了memoryService和memoryFile后，我们就可以在Client端访问Server端创建的匿名共享内存了。点击Read按钮时，就通过memoryFile的readBytes接口把共享内存中的整数读出来，并显示在文本框中；点击Write按钮时，就通过memoryService这个代理类的setVal接口来调用MemoryService的本地实现类的setVal服务，从而把文本框中的数值写到Server端创建的匿名共享内存中去；点击Clear按钮时，就会清空文本框的内容。这样，我们就可以通过Read和Write按钮来验证我们是否在Client和Server两个进程中实现内存共享了。

       现在，我们再来看看Client界面的配置文件，它定义在res/layout/main.xml文件中：

 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:gravity="center">
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/value">
        </TextView>
        <EditText
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:id="@+id/edit_value"
            android:hint="@string/hint">
        </EditText>
    </LinearLayout>
     <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:gravity="center">
        <Button
            android:id="@+id/button_read"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/read">
        </Button>
        <Button
            android:id="@+id/button_write"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/write">
        </Button>
        <Button
            android:id="@+id/button_clear"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/clear">
        </Button>
    </LinearLayout>
</LinearLayout>