本文分析了从Java层到Native层分析了Bitmap对象的创建过程。
Bitmap在Android4.0之后将Bitmap的内存从Native层转移到Java层了。
在Java层，创建一个Bitmap对象需要使用Bitmap的createBitmap的系列方法,Bitmap通过下面的方法来时创建Bitmap对象。1
private static Bitmap createBitmap(DisplayMetrics display, int width, int height,
Config config, boolean hasAlpha) {
if (width <= 0 || height <= 0) {
throw new IllegalArgumentException("width and height must be > 0");
}
Bitmap bm = nativeCreate(null, 0, width, width, height, config.nativeInt, true);
if (display != null) {
bm.mDensity = display.densityDpi;
}
bm.setHasAlpha(hasAlpha);
if (config == Config.ARGB_8888 && !hasAlpha) {
nativeErase(bm.mFinalizer.mNativeBitmap, 0xff000000);
}
// No need to initialize the bitmap to zeroes with other configs;
// it is backed by a VM byte array which is by definition preinitialized
// to all zeroes.
return bm;
}
上面代码位于framework/base/core/graphics/java/android/graphics/Bitmap.java
在createBitmap方法中通过nativeCreate方法来创建Bitmap对象，nativeCreate方法的申明如下：1
private static native Bitmap nativeCreate(int[] colors, int offset,
int stride, int width, int height,
int nativeConfig, boolean mutable);
nativeCreate的参数colors是像素点颜色数组，offset是数组的起始偏移量，stride是每个像素颜色的跨越值，width是bitmap 的宽度值，height是bitmap 的高度值，nativeConfig是Config的int值，Config是一个枚举类型，代码下面列出，nativeConfigInt也就是config对象的int型表示。1
public enum Config {
// these native values must match up with the enum in SkBitmap.h
ALPHA_8 (1),
RGB_565 (3),
@Deprecated
ARGB_4444 (4),
ARGB_8888 (5);
final int nativeInt;
private static Config sConfigs[] = {
null, ALPHA_8, null, RGB_565, ARGB_4444, ARGB_8888
};
Config(int ni) {
this.nativeInt = ni;
}
static Config nativeToConfig(int ni) {
return sConfigs[ni];
}
}
nativeCreate在Native层对应的方法是framework/base/core/jni/android/graphics/Bitmap.cpp文件中的Bitmap_creator方法中。1
static jobject Bitmap_creator(JNIEnv* env, jobject, jintArray jColors,
jint offset, jint stride, jint width, jint height,
jint configHandle, jboolean isMutable) {
SkColorType colorType = GraphicsJNI::legacyBitmapConfigToColorType(configHandle);
if (NULL != jColors) {
size_t n = env->GetArrayLength(jColors);
if (n < SkAbs32(stride) * (size_t)height) {
doThrowAIOOBE(env);
return NULL;
}
}
// ARGB_4444 is a deprecated format, convert automatically to 8888
if (colorType == kARGB_4444_SkColorType) {
colorType = kN32_SkColorType;
}
SkBitmap bitmap;
bitmap.setInfo(SkImageInfo::Make(width, height, colorType, kPremul_SkAlphaType));
Bitmap* nativeBitmap = GraphicsJNI::allocateJavaPixelRef(env, &bitmap, NULL);
if (!nativeBitmap) {
return NULL;
}
if (jColors != NULL) {
GraphicsJNI::SetPixels(env, jColors, offset, stride,
0, 0, width, height, bitmap);
}
return GraphicsJNI::createBitmap(env, nativeBitmap,
getPremulBitmapCreateFlags(isMutable));
}
这里首先调用GraphicsJNI::legacyBitmapConfigToColorType将configHandle转换成SkColorType,configHandle是Java层的Config对象的int型表示，legacyBitmapConfigToColorType函数如下：1
SkColorType GraphicsJNI::legacyBitmapConfigToColorType(jint legacyConfig) {
const uint8_t gConfig2ColorType[] = {
kUnknown_SkColorType,
kAlpha_8_SkColorType,
kIndex_8_SkColorType,
kRGB_565_SkColorType,
kARGB_4444_SkColorType,
kN32_SkColorType
};
if (legacyConfig < 0 || legacyConfig > kLastEnum_LegacyBitmapConfig) {
legacyConfig = kNo_LegacyBitmapConfig;
}
return static_cast<SkColorType>(gConfig2ColorType[legacyConfig]);
}
之后colors数组不为NULL的话，检查colors数组的合法性，如果colors的长度小于stride*height，说明colors数组中要跳过的数大于数组的长度，也就是colors数组中没有合法的像素值。如果colors数组是合法的，则继续执行。1
SkBitmap bitmap;
bitmap.setInfo(SkImageInfo::Make(width, height, colorType, kPremul_SkAlphaType));
1 | SkImageInfo() : fWidth(0) , fHeight(0) , fColorType(kUnknown_SkColorType) , fAlphaType(kUnknown_SkAlphaType) , fProfileType(kLinear_SkColorProfileType) {} static SkImageInfo Make(int width, int height, SkColorType ct, SkAlphaType at, SkColorProfileType pt = kLinear_SkColorProfileType) { return SkImageInfo(width, height, ct, at, pt); } |
SkImageInfo定义在external/skia/include/core/SkImageInfo.h文件中
创建一个SkBitmap对象，调用SkImageInfo::Make方法通过从Java层传下来的width，height以及上面转换得到的colorType，以及kPremul_SkAlphaType来得到一个SkBitmapInfo对象，kPremul_SkAlphaType是一个SkAlphaType类型，1
enum SkAlphaType {
//All pixels are stored as opaque
kUnknown_SkAlphaType,
kOpaque_SkAlphaType,
kPremul_SkAlphaType,
kUnpremul_SkAlphaType,
kLastEnum_SkAlphaType = kUnpremul_SkAlphaType
};
之后调用SkBitmap.setInfo将得到SkImageInfo记录下来。调用GraphicsJNI::allocateJavaPixelRef方法，此方法是用于在Java层给Bitmap对象分配内存。该方法实现位于framework/base/jni/android/graphics/Graphics.cpp文件中：1
android::Bitmap* GraphicsJNI::allocateJavaPixelRef(JNIEnv* env, SkBitmap* bitmap,
SkColorTable* ctable) {
const SkImageInfo& info = bitmap->info();
if (info.fColorType == kUnknown_SkColorType) {
doThrowIAE(env, "unknown bitmap configuration");
return NULL;
}
size_t size;
if (!computeAllocationSize(*bitmap, &size)) {
return NULL;
}
// we must respect the rowBytes value already set on the bitmap instead of
// attempting to compute our own.
const size_t rowBytes = bitmap->rowBytes();
jbyteArray arrayObj = (jbyteArray) env->CallObjectMethod(gVMRuntime,
gVMRuntime_newNonMovableArray,
gByte_class, size);
if (env->ExceptionCheck() != 0) {
return NULL;
}
SkASSERT(arrayObj);
jbyte* addr = (jbyte*) env->CallLongMethod(gVMRuntime, gVMRuntime_addressOf, arrayObj);
if (env->ExceptionCheck() != 0) {
return NULL;
}
SkASSERT(addr);
android::Bitmap* wrapper = new android::Bitmap(env, arrayObj, (void*) addr,
info, rowBytes, ctable);
wrapper->getSkBitmap(bitmap);
// since we're already allocated, we lockPixels right away
// HeapAllocator behaves this way too
bitmap->lockPixels();
return wrapper;
}
在这个方法中，首先info的fColorType和kUnknown_SkColorType相等的话，表示要创建的Bitmap的colorType是错误的，方法返回。否则继续执行，计算需要分配的大小，如果大小等于0，就不需要创建，直接返回。不等于0的话，通过调用jni的CallObjectMethod来调用gVimRuntime的gVMRuntime_newNonMovableArray方法来创建一个数组，这个数组类型和长度分别用gByte_class和size表示。CallObjectMethod函数返回一个jbyteArray，此时，在Java层已经创建了一个长度为size的byte数组。
数组创建完毕后，通过调用jni的CallLongMethod方法调用gVMRuntime对象的gVMRuntime_addressOf方法来获取上面的到arrayObj的数组地址。
得到数组地址后，创建一个Native层的Bitmap对象wrapper，调用wrapper的getSkBitmap方法为bitmap设置了一些参数。1
void Bitmap::getSkBitmap(SkBitmap* outBitmap) {
assertValid();
android::AutoMutex _lock(mLock);
// Safe because mPixelRef is a WrappedPixelRef type, otherwise rowBytes()
// would require locking the pixels first.
outBitmap->setInfo(mPixelRef->info(), mPixelRef->rowBytes());
outBitmap->setPixelRef(refPixelRefLocked())->unref();
outBitmap->setHasHardwareMipMap(hasHardwareMipMap());
}
调用SkBitmap对象bitmap的lockPixels方法锁定像素数据后，GraphicsJNI::allocateJavaPixelRef函数返回wrapper对象，回到Bitmap_creator方法。1
static jobject Bitmap_creator(JNIEnv* env, jobject, jintArray jColors,
jint offset, jint stride, jint width, jint height,
jint configHandle, jboolean isMutable) {
......
Bitmap* nativeBitmap = GraphicsJNI::allocateJavaPixelRef(env, &bitmap, NULL);
if (!nativeBitmap) {
return NULL;
}
if (jColors != NULL) {
GraphicsJNI::SetPixels(env, jColors, offset, stride,
0, 0, width, height, bitmap);
}
return GraphicsJNI::createBitmap(env, nativeBitmap,
getPremulBitmapCreateFlags(isMutable));
} getPremulBitmapCreateFlags(isMutable));
如果GraphicsJNI::allocateJavaPixelRef返回的Native 层的Bitmap对象为NULL，函数返回。否则，函数继续执行。
如果jColors不为NULL，那么将jColors中表示的像素值传递给前面创建在Java层的数组中。
调用GraphicsJNI::createBitmap来创建Java层的Bitmap对象。函数定义在framework/base/jni/android/graphics/Graphics.cpp中1
jobject GraphicsJNI::createBitmap(JNIEnv* env, android::Bitmap* bitmap,
int bitmapCreateFlags, jbyteArray ninePatchChunk, jobject ninePatchInsets,
int density) {
bool isMutable = bitmapCreateFlags & kBitmapCreateFlag_Mutable;
bool isPremultiplied = bitmapCreateFlags & kBitmapCreateFlag_Premultiplied;
// The caller needs to have already set the alpha type properly, so the
// native SkBitmap stays in sync with the Java Bitmap.
assert_premultiplied(bitmap->info(), isPremultiplied);
jobject obj = env->NewObject(gBitmap_class, gBitmap_constructorMethodID,
reinterpret_cast<jlong>(bitmap), bitmap->javaByteArray(),
bitmap->width(), bitmap->height(), density, isMutable, isPremultiplied,
ninePatchChunk, ninePatchInsets);
hasException(env); // For the side effect of logging.
return obj;
}
这里主要是调用jni的NewObject来创建了一个Java层的Bitmap对象，gBitmap_class代表的Java层的Bitmap类名1
int register_android_graphics_Graphics(JNIEnv* env)
{
......
gBitmap_class = make_globalref(env, "android/graphics/Bitmap");
......
｝
定义在framework/base/jni/android/graphics/Graphics.cpp文件中