虽然是老技术了，不过还是值得学习一下。
例子中截屏作为一个开机启动的后台service，在需要截屏的任何地方(系统servic或其他APK)，发送 Intent "com.android.CAPTURE_SCREEN" 即可
我的测试版本是2.3.4，其它不保证能用 源码：ScreenCap

示例代码在:
frameworksbaseservicessurfaceflingertestsscreencapscreencap.cpp

/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include <utils/Log.h>
#include <binder/IPCThreadState.h>
#include <binder/ProcessState.h>
#include <binder/IServiceManager.h>
#include <binder/IMemory.h>
#include <surfaceflinger/ISurfaceComposer.h>
#include <SkImageEncoder.h>
#include <SkBitmap.h>
using namespace android;
int main(int argc, char** argv)
{
    if (argc != 2) {
        printf("usage: %s pathn", argv[0]);
        exit(0);
    }
    const String16 name("SurfaceFlinger");
    sp<ISurfaceComposer> composer;
    getService(name, &composer);
    sp<IMemoryHeap> heap;
    uint32_t w, h;
    PixelFormat f;
    status_t err = composer->captureScreen(0, &heap, &w, &h, &f, 0, 0);
    if (err != NO_ERROR) {
        fprintf(stderr, "screen capture failed: %sn", strerror(-err));
        exit(0);
    }
    printf("screen capture success: w=%u, h=%u, pixels=%pn",
            w, h, heap->getBase());
    printf("saving file as PNG in %s ...n", argv[1]);
    SkBitmap b;
    b.setConfig(SkBitmap::kARGB_8888_Config, w, h);
    b.setPixels(heap->getBase());
    SkImageEncoder::EncodeFile(argv[1], b,
            SkImageEncoder::kPNG_Type, SkImageEncoder::kDefaultQuality);
    return 0;
}

编译后生成 /system/bin/test-screencap
测试时终端输入 test-screencap /mnt/sdcard/scapxx.png
可以看到SD卡生成截屏文件scapxx.png，默认大小为屏幕分辨率
如果想把这个功能写到自己的应用里
写个JNI，参考上面代码即可，但记得权限声明

<uses-permission android:name="android.permission.READ_FRAME_BUFFER" />

jni/com_android_ScreenCap_ScreenCapNative.cpp

/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#define LOG_TAG "ScreenCapNative-JNI"
//#define LOG_NDEBUG 0
#include "JNIHelp.h"
#include "jni.h"
#include <utils/Log.h>
#include <binder/IPCThreadState.h>
#include <binder/ProcessState.h>
#include <binder/IServiceManager.h>
#include <binder/IMemory.h>
#include <surfaceflinger/ISurfaceComposer.h>
#include <SkImageEncoder.h>
#include <SkBitmap.h>
namespace android {	
static void android_ScreenCap_ScreenCapNative_nativeCaptureScreen(JNIEnv* env,
        jobject obj, jstring file) 
{
	const char *file_path = env->GetStringUTFChars(file, NULL);
    const String16 name("SurfaceFlinger");
    sp<ISurfaceComposer> composer;
    getService(name, &composer);
    sp<IMemoryHeap> heap;
    uint32_t w, h;
    PixelFormat f;
    status_t err = composer->captureScreen(0, &heap, &w, &h, &f, 0, 0);
    if (err != NO_ERROR) {
        LOGE("screen capture failed: %sn", strerror(-err));
        exit(0);
    }
    LOGD("screen capture success: w=%u, h=%u, pixels=%pn",
            w, h, heap->getBase());
    LOGD("saving file as PNG in %s ...n", file_path);
    SkBitmap b;
    b.setConfig(SkBitmap::kARGB_8888_Config, w, h);
    b.setPixels(heap->getBase());
    SkImageEncoder::EncodeFile(file_path, b,
            SkImageEncoder::kPNG_Type, SkImageEncoder::kDefaultQuality);        	
}
static JNINativeMethod gScreenCapNativeMethods[] = {
    /* name, signature, funcPtr */
    { "nativeCaptureScreen", "(Ljava/lang/String;)V",
            (void*) android_ScreenCap_ScreenCapNative_nativeCaptureScreen },            
};
	
int register_android_ScreenCap_ScreenCapNative(JNIEnv* env) {
    int res = jniRegisterNativeMethods(env, "com/android/ScreenCap/ScreenCapNative",
            gScreenCapNativeMethods, NELEM(gScreenCapNativeMethods));
    LOG_FATAL_IF(res < 0, "Unable to register native methods.");
    return 0;
}	
	
extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JNIEnv* env = NULL;
    jint result = -1;
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("GetEnv failed!");
        return result;
    }
    LOG_ASSERT(env, "Could not retrieve the env!");
    register_android_ScreenCap_ScreenCapNative(env);
    return JNI_VERSION_1_4;
}	
	
} /* namespace android */

jni/Android.mk

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES := com_android_ScreenCap_ScreenCapNative.cpp
LOCAL_C_INCLUDES += 
	$(JNI_H_INCLUDE)
LOCAL_SHARED_LIBRARIES := 
	libnativehelper 
	libcutils 
	libutils 
	libbinder 
	libskia 
    libui 
    libsurfaceflinger_client
LOCAL_C_INCLUDES += 
	external/skia/include/core 
	external/skia/include/effects 
	external/skia/include/images 
	external/skia/src/ports 
	external/skia/include/utils
LOCAL_MODULE:= libscreencapjni
LOCAL_MODULE_TAGS := optional
LOCAL_PRELINK_MODULE := false
include $(BUILD_SHARED_LIBRARY)    

src/com/android/ScreenCap/ScreenCapNative.java

package com.android.ScreenCap;
import java.io.File;
import android.os.Environment;
public class ScreenCapNative {
	
	static {
    	System.loadLibrary("screencapjni");
    };
    
	private native static void nativeCaptureScreen(String file);
	
	public static String startCaptureScreen() {
    	String file_name = getUniqueFileName();
    	if (file_name != null) {
    		nativeCaptureScreen(file_name);
    	}
    	return file_name;
    }
    
   	private static String getUniqueFileName() {
    	File file = null;
    	if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            file = createUniqueFile(Environment.getExternalStorageDirectory(),
                    "ScreenCap.PNG");
        } else if (Environment.getInternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            file = createUniqueFile(Environment.getInternalStorageDirectory(),
                    "ScreenCap.PNG");                
        }
        
    	return (file != null) ? file.getAbsolutePath() : null;
    	
    }
    
	private static File createUniqueFile(File directory, String filename) {
        File file = new File(directory, filename);
        if (!file.exists()) {
            return file;
        }
        // Get the extension of the file, if any.
        int index = filename.lastIndexOf('.');
        String format;
        if (index != -1) {
            String name = filename.substring(0, index);
            String extension = filename.substring(index);
            format = name + "-%d" + extension;
        }
        else {
            format = filename + "-%d.PNG";
        }
        for (int i = 2; i < Integer.MAX_VALUE; i++) {
            file = new File(directory, String.format(format, i));
            if (!file.exists()) {
                return file;
            }
        }
        return null;
    }  	
		
}

可以在你的Acitvity或Service里面调用 ScreenCapNative.startCaptureScreen();来截屏
4.0源码已经公布了，大家不要花时间研究2.3的截屏了
当然最好是系统级实现，响应某个组合键来截屏，像Android4.0

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