我得到了一个java.lang.OutOfMemoryError：Java堆空间，即使使用GSON Streaming也是如此。
{"result":"OK","base64":"JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC...."}
base64最长可达200Mb。 GSON占用的内存要多得多，（3GB）当我尝试将base64存储在一个变量中时，我得到一个：
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:2367)
at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:130)
at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:114)
at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:535)
at java.lang.StringBuilder.append(StringBuilder.java:204)
at com.google.gson.stream.JsonReader.nextQuotedValue(JsonReader.java:1014)
at com.google.gson.stream.JsonReader.nextString(JsonReader.java:815)
处理这种领域的最佳方法是什么？
答案 0 :(得分：4)
你获得OutOfMemoryError的原因是GSON nextString()返回一个字符串，该字符串是在使用StringBuilder构建一个非常大的字符串时聚合的。当您遇到这样的问题时，您必须处理中间数据，因为没有其他选择。不幸的是，GSON不会让你以任何方式处理大量的文字。
不确定是否可以更改响应有效负载，但如果不能，则可能需要实现自己的JSON阅读器，或“破解”现有的JsonReader以使其以流式方式工作。下面的示例基于GSON 2.5并大量使用反射，因为JsonReader非常小心地隐藏了它的状态。
final class EnhancedGson25JsonReader
extends JsonReader {
// A listener to accept the internal character buffers.
// Accepting a single string built on such buffers is total memory waste as well.
interface ISlicedStringListener {
void accept(char[] buffer, int start, int length)
throws IOException;
}
// The constants can be just copied
/** @see JsonReader#PEEKED_NONE */
private static final int PEEKED_NONE = 0;
/** @see JsonReader#PEEKED_SINGLE_QUOTED */
private static final int PEEKED_SINGLE_QUOTED = 8;
/** @see JsonReader#PEEKED_DOUBLE_QUOTED */
private static final int PEEKED_DOUBLE_QUOTED = 9;
// Here is a bunch of spies made to "spy" for the parent's class state
private final FieldSpy<Integer> peeked;
private final MethodSpy<Integer> doPeek;
private final MethodSpy<Integer> getLineNumber;
private final MethodSpy<Integer> getColumnNumber;
private final FieldSpy<char[]> buffer;
private final FieldSpy<Integer> pos;
private final FieldSpy<Integer> limit;
private final MethodSpy<Character> readEscapeCharacter;
private final FieldSpy<Integer> lineNumber;
private final FieldSpy<Integer> lineStart;
private final MethodSpy<Boolean> fillBuffer;
private final MethodSpy<IOException> syntaxError;
private final FieldSpy<Integer> stackSize;
private final FieldSpy<int[]> pathIndices;
private EnhancedJsonReader(final Reader reader)
throws NoSuchFieldException, NoSuchMethodException {
super(reader);
peeked = spyField(JsonReader.class, this, "peeked");
doPeek = spyMethod(JsonReader.class, this, "doPeek");
getLineNumber = spyMethod(JsonReader.class, this, "getLineNumber");
getColumnNumber = spyMethod(JsonReader.class, this, "getColumnNumber");
buffer = spyField(JsonReader.class, this, "buffer");
pos = spyField(JsonReader.class, this, "pos");
limit = spyField(JsonReader.class, this, "limit");
readEscapeCharacter = spyMethod(JsonReader.class, this, "readEscapeCharacter");
lineNumber = spyField(JsonReader.class, this, "lineNumber");
lineStart = spyField(JsonReader.class, this, "lineStart");
fillBuffer = spyMethod(JsonReader.class, this, "fillBuffer", int.class);
syntaxError = spyMethod(JsonReader.class, this, "syntaxError", String.class);
stackSize = spyField(JsonReader.class, this, "stackSize");
pathIndices = spyField(JsonReader.class, this, "pathIndices");
}
static EnhancedJsonReader getEnhancedGson25JsonReader(final Reader reader) {
try {
return new EnhancedJsonReader(reader);
} catch ( final NoSuchFieldException | NoSuchMethodException ex ) {
throw new RuntimeException(ex);
}
}
// This method has been copied and reworked from the nextString() implementation
void nextSlicedString(final ISlicedStringListener listener)
throws IOException {
int p = peeked.get();
if ( p == PEEKED_NONE ) {
p = doPeek.get();
}
switch ( p ) {
case PEEKED_SINGLE_QUOTED:
nextQuotedSlicedValue('\'', listener);
break;
case PEEKED_DOUBLE_QUOTED:
nextQuotedSlicedValue('"', listener);
break;
default:
throw new IllegalStateException("Expected a string but was " + peek()
+ " at line " + getLineNumber.get()
+ " column " + getColumnNumber.get()
+ " path " + getPath()
);
}
peeked.accept(PEEKED_NONE);
pathIndices.get()[stackSize.get() - 1]++;
}
// The following method is also a copy-paste that was patched for the "spies".
// It's, in principle, the same as the source one, but it has one more buffer singleCharBuffer
// in order not to add another method to the ISlicedStringListener interface (enjoy lamdbas as much as possible).
// Note that the main difference between these two methods is that this one
// does not aggregate a single string value, but just delegates the internal
// buffers to call-sites, so the latter ones might do anything with the buffers.
/**
* @see JsonReader#nextQuotedValue(char)
*/
private void nextQuotedSlicedValue(final char quote, final ISlicedStringListener listener)
throws IOException {
final char[] buffer = this.buffer.get();
final char[] singleCharBuffer = new char[1];
while ( true ) {
int p = pos.get();
int l = limit.get();
int start = p;
while ( p < l ) {
final int c = buffer[p++];
if ( c == quote ) {
pos.accept(p);
listener.accept(buffer, start, p - start - 1);
return;
} else if ( c == '\\' ) {
pos.accept(p);
listener.accept(buffer, start, p - start - 1);
singleCharBuffer[0] = readEscapeCharacter.get();
listener.accept(singleCharBuffer, 0, 1);
p = pos.get();
l = limit.get();
start = p;
} else if ( c == '\n' ) {
lineNumber.accept(lineNumber.get() + 1);
lineStart.accept(p);
}
}
listener.accept(buffer, start, p - start);
pos.accept(p);
if ( !fillBuffer.apply(just1) ) {
throw syntaxError.apply(justUnterminatedString);
}
}
}
// Save some memory
private static final Object[] just1 = { 1 };
private static final Object[] justUnterminatedString = { "Unterminated string" };
}
final class FieldSpy<T>
implements Supplier<T>, Consumer<T> {
private final Object instance;
private final Field field;
private FieldSpy(final Object instance, final Field field) {
this.instance = instance;
this.field = field;
}
static <T> FieldSpy<T> spyField(final Class<?> declaringClass, final Object instance, final String fieldName)
throws NoSuchFieldException {
final Field field = declaringClass.getDeclaredField(fieldName);
field.setAccessible(true);
return new FieldSpy<>(instance, field);
}
@Override
public T get() {
try {
@SuppressWarnings("unchecked")
final T value = (T) field.get(instance);
return value;
} catch ( final IllegalAccessException ex ) {
throw new RuntimeException(ex);
}
}
@Override
public void accept(final T value) {
try {
field.set(instance, value);
} catch ( final IllegalAccessException ex ) {
throw new RuntimeException(ex);
}
}
}
final class MethodSpy<T>
implements Function<Object[], T>, Supplier<T> {
private static final Object[] emptyObjectArray = {};
private final Object instance;
private final Method method;
private MethodSpy(final Object instance, final Method method) {
this.instance = instance;
this.method = method;
}
static <T> MethodSpy<T> spyMethod(final Class<?> declaringClass, final Object instance, final String methodName, final Class<?>... parameterTypes)
throws NoSuchMethodException {
final Method method = declaringClass.getDeclaredMethod(methodName, parameterTypes);
method.setAccessible(true);
return new MethodSpy<>(instance, method);
}
@Override
public T get() {
// my javac generates useless new Object[0] if no args passed
return apply(emptyObjectArray);
}
@Override
public T apply(final Object[] arguments) {
try {
@SuppressWarnings("unchecked")
final T value = (T) method.invoke(instance, arguments);
return value;
} catch ( final IllegalAccessException | InvocationTargetException ex ) {
throw new RuntimeException(ex);
}
}
}
这是一个使用该方法读取巨大的JSON并将其字符串值重定向到另一个文件的演示。
public static void main(final String... args)
throws IOException {
try ( final EnhancedGson25JsonReader input = getEnhancedGson25JsonReader(new InputStreamReader(new FileInputStream("./huge.json")));
final Writer output = new OutputStreamWriter(new BufferedOutputStream(new FileOutputStream("./huge.json.STRINGS"))) ) {
while ( input.hasNext() ) {
final JsonToken token = input.peek();
switch ( token ) {
case BEGIN_OBJECT:
input.beginObject();
break;
case NAME:
input.nextName();
break;
case STRING:
input.nextSlicedString(output::write);
break;
default:
throw new AssertionError(token);
}
}
}
}
我成功将上面的字段提取到文件中。输入文件长度为544 MB（ 570 425 371 字节），并由以下JSON块生成：
{"result":"OK","base64":" JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC× 16777216 （2 ^ 24）"} 结果是（因为我只是将所有字符串重定向到文件）：
OK JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC× 16777216 （2 ^ 24）我认为你面临一个非常有趣的问题。从GSON团队那里得到一些可能的API增强反馈会很好。