Building a specific piece of Android platform?

I have been trying to build only the "/libcore" directory of the Android platform. When I开发者_高级运维 try mmm libcore I end up with the following output:

============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=2.1-update1
TARGET_PRODUCT=generic
TARGET_BUILD_VARIANT=eng
TARGET_SIMULATOR=false
TARGET_BUILD_TYPE=release
TARGET_ARCH=arm
HOST_ARCH=x86
HOST_OS=linux
HOST_BUILD_TYPE=release
BUILD_ID=ECLAIR
============================================
make: Entering directory `/home/chris/android/platform'
target Prebuilt:  (out/target/product/generic/system/etc/security/cacerts.bks)
host Prebuilt: run-core-tests-on-ri (out/host/linux-x86/obj/EXECUTABLES/run-core-tests-on-ri_intermediates/run-core-tests-on-ri)
target Prebuilt: run-core-tests (out/target/product/generic/obj/EXECUTABLES/run-core-tests_intermediates/run-core-tests)
Copy: out/target/product/generic/system/etc/apns-conf.xml
Copying: out/target/common/obj/JAVA_LIBRARIES/core_intermediates/classes-full-debug.jar
Copying: out/target/common/obj/JAVA_LIBRARIES/core-tests_intermediates/classes-full-debug.jar
/bin/bash: jar: command not found
make: *** [out/host/common/core-tests.jar] Error 127
make: *** Deleting file `out/host/common/core-tests.jar'
make: Leaving directory `/home/chris/android/platform'

Does anyone have any suggestions on what Error 127 is, or another method I can go about building "libcore" without having to build the entire platform again?

Thanks, Chris

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It says "jar: command not found"

Are you sure you have a Java SDK installed?